Hardware question: CPU upgrade, temperature and TDP

So it's not specifically a Win 10 question - but perhaps you can help me? Also, moderators, feel free to move or delete...

So I inherited my Mom's HP Probook 4540s. Works fine, not super powerful, looks pretty sleek. May keep it for houseguests to use if they visit. May sell it. Goofing around with it now. I will say that fiddling with the guts of the thing gives one respect for the old Thinkpads. The fan casing is pretty flimsy. But its pretty easy to work on.

Came with 4gb (1 stick! On a dual channel machine!) and a 3210M cpu. Also had an SSD. I went a little Tim Allen on it. I put 8Gb in, which is the nameplate limit, but I could have put 16gb in (would have to adjust the set limit in msconfig, no big deal). The 3210 is pretty weak. I noticed that a 3720QM has the same pinouts, and is much much faster (like 3x). Long story short, I've found that with a 3720qm the machine boots and works fine. One point I had seen, but figured I'd evaluate after install, was a TDP of 45W for the 3720qm vs 35W for all of the CPUs offered in the 4540s. So I knew the temperature would be a bit higher. It's normally around 60°C, ranging from about 47°, spiking to 70° and rarely 80°. Junction temp for this CPU is 105°C. I felt pretty confident about using a 3720, because I'd read that someone else had.

But is the temperature too high? I could put the original CPU in and the thing would work fine, I guess. But I want more power. I also could put in a 3612qm, but that's more money. I'm tending towards replacing the original CPU, and selling the machine and the 3720qm.

Comments, suggestions? Thanks!

So I inherited my Mom's Probook 4540s. It's a nice-looking design, but there were a couple of issues. I upgraded the wi-fi to dual band (AC7260). I upgraded the memory from 4 (initially a single SODIMM, so only one memory channel used!) to 8Gb (2x 4Gb). 8Gb is the published limit, and initially I believed that this limit was artificially set by HP because some CPUs offered with the 4540s only supported 8Gb. So to avoid confusion, the thinking went, they just specifed an 8Gb limit. But a cursory investigation shows that these machines could handle 16Gb. So, maybe the limit was marketing, maybe battery life (?) or some other issue. But if the limit is artificial, you can go to msconfig>boot tab>advanced setting and unclick the maximum memory box. Close that up, shut down, add 16gb of memory, reboot, and (for housekeeping sake) go to msconfig>boot>advanced. The maximum memory should show 16gb. You can reclick the button and should be good to go. The drive was already SSD. Cool!

But that's not the exciting news. The 3210m cpu in the machine is dual, not quad core. There are lies, damned lies, and benchmarks, but the benchmark numbers seem to indicate that the 3210qm runs about 2.3 times slower than a pin-compatible 3720qm quad core. WTH, I tried it. I swapped the 3210 out and put a 3720qm in. The only issue was TDP. The 3720 has a thermal design power (TDP) rating of 45W, the 3210 of 35W. I didn't note the CPU temperature before the switch, but afterwards it was getting near 80C. That's pretty high - of concern. But the CPU Passmark scores were indeed 3x higher. In discussions on other forums a solution was suggested. Use a larger, higher capacity heatsink from a discrete GPU unit, making sure that any electrical contact surfaces are insulated. For 15 bucks I got one. It's very very easy to work in the back of a 4540s. The new heatsink was plug and play. The critical result: max temperature was 11°C lower (I used Passmark Performance Test as the stress test)!

You take full responsibility if you try the following. This is NOT advocated by HP and almost certainly will void your warranty and might even cause HP to refuse to service your PC. Your choice, your responsibility.

0) Obtain new CPU (see CPU updates for compatibility) and used discrete graphics heat sink (this heat sink has a larger tube, and an additional pad that interfaces to the discrete GPU). The 3720qm worked and is probably the best performance/cost tradeoff. A 3820qm or a 3840qm should work. The 3920XM might be a bridge too far: it has a TDP of 55W, iirc.
1) remove battery
2) Press the on button to discharge as much of the stored energy as possible. In some machines, you'll see the fan spin a bit when you do this!
3) remove back cover (may require one screw removal - mine's gone)
4) MAKE SURE YOU ARE GROUNDED SO YOU DON'T SHORT OUT THE MACHINE (you may want a grounding wrist strap for this), and then
5) remove the three screws holding the metal cpu cover on.
6) slide the cover in the direction indicated and pivot it out
7) unscrew the last screw holding the fan (near circuit board level).
8) Unplug and remove the fan
9) Remove the heatsink and (away from your laptop) clean it. Avoid old thermal paste crumbs in the macine. The final step in cleaning is an isopropanol wipe.
10) replace the CPU (don't know if I'd buy a 3840qm but if I had one I'd sure try it!) Turn the retaining screw 180, remove the old, drop in the new, and turn the retaining screw back to original position. Make sure that you don't get any heat transfer compound into the pin grid socket.
11) Make sure that the new CPU has all old heat transfer paste removed.
12) put new neat transfer paste on the CPU (see web for indication of how much you put on).
13) Using Kapton tape, insulate the GPU heat transfer pad. You don't want the pad (copper) to touch the unused GPU ball grid array causing a short. I bought 5 mil tape for this. If you really want to get fancy, you could cut most of the GPU pad off with a Dremel tool and an abrasive disc (smooth the edges with a file if you do). I didn't.
14) Install the heat sink. Tighten the screws to the CPU pad. The numbers on the heat sink seem to indicate an order (1234). I used that order.
15) reinstall the fan, plug it in, and tighten one screw to fasten it down.
16) reinstall the metal CPU shield, using the original 3 screws.
17) put the back on, and the battery in
18) boot up and test!

This is not a monumental game changer. But it will extend the life of this laptop a few more years. In my opinion, the key upgrades probably don't include a CPU. But to keep an old laptop running in Win 10:

1) Get memory to 4 or 8Gb, and if the CPU is dual channel, use two SODIMMs to do that. For a 4540s, at a minimum use the specified DDR3 PC3-10600 SDRAM (1333 MHz). But remember, if you have (or can find cheap) PC3-12800 (1600MHz) those should work too.
2) Change from a physical hard drive to a solid state drive. They're cheap.
3) Change the wifi card to a dual band (I used an intel AC7260 - about 12 bucks).
The CPU change I mention above is the last upgrade to do. [NOTE: in some laptops, the CPU is permanently attached to the motherboard using a ball gride array (BGA) and not a pin grid array (PGA), and if this is so, you can't upgrade easily). It's more intrusive and time intensive, you have a great risk of breaking something, you void any warranty, and HP might (for good reason) refuse to look at your machine if you break it.

For me, it was fun, and the machine screams now. Paradoxically, I'm going to clean it up and sell it on eBay. The fun was in the journey, not the destination. Thought some would find this interesting/useful. There may be other machines with different capacity heatsinks that allow a CPU upgrade.