The IRST drivers usually should be preinstalled before the OS. With my PC Windows 10 had the new SATA Controller drivers (IRST Drivers). Also need to make sure have Chipset driver software installed.Pretty sure it's all Intel on your system.
As far as what you saw from 250GB now showing 232GB same with mine and explained by WordMan here. If have a lot of software might of needed to think 500GB SSD then or put some software on the HDD. As far as games they will definitely run better on a SSD too. I have 2 SSD, 1 for OS and 1 for games, then a HDD for backup.
Let me know if still want to tackle AHCI, but as i said and others it will work o.k. without it, to me if you can do it i would, up to you.
Go in to your properties, get some information about the driver version, date, and date it was installed, who installed it, most likely it was MS, write it down. Then go to intel's web site, driver section. should be new drivers listed for your controller listed.
You don't do this much do ya..... I've been doing this for my computers for ages. Sometimes I get some from HP sometimes I go to Intel. Thing is Gary, even if you find one, download it, install it, when we get the big update Nov 2nd, I wouldn't be surprised if MS removes them again!!! Maybe not.....but that's why we make our image's.
Bottom line Gary, if you don't feel cool doing it....or checking Intel's site.....don't. I just thought it might be worth a shot.
Just be careful with installing drivers even if from the source like Intel read the release notes carefully, make sure it matches your chipset family. Always try the old right click search online for drivers in device manager. Wrong driver render system unbootable, then will be back to square one.
Oh yeah one more thing Gary, if you find a driver update for Intel's controller and do download it, Intel's software is smart enough not to update a driver that is the wrong driver. It won't install an incorrect driver, most of the time.