Would a ram with higher MHz bus than motherboard's overclock better ?

Hi, just mentioning "laptop" precludes most of overclocking because of poor and locked settings in BIOS. In theory, DDR3 is starting with 1066MHz and everything above constitutes OC. In modern RAM that is done using XMP profile written in modules internal controller which BIOS is supposed to recognize and set RAM accordingly.
Other than using XMP. RAM could be OC-ed in several ways. One is by just adjusting it's frequency but that also means it will raise Cas Latency diminishing overall performance gains. Voltage may also have to be raised for reliability which is poor option for battery life and generates more heat, both detrimental to laptops.
Other option is to change RAM divider which is not adjustable in most modern desktop boards let alone portables.
Raising BCLK/FSB is another way but that also raises CPU frequency and frequencies of other buses (PCI, PCIe and SATA) and if it's exceeded too much can also draw more power and produce more heat. VRM in laptops and in desktop boards may not be up to task, reducing even more heat and eventually throttling down whole system producing bad instability and worse performance under a load.
Actual memory controller is in CPU and may not take it kindly to raising frequency, protesting in various was. Every CPU has some manufacturer's determined "natural" frequency and going over it (if possible at all) produces very little gains. Raising RAM frequency without also raising CPU frequency by one step (like 1600 to 1866MHz) may add only couple of % in memory performance which also features only about 1% in overall system performance in best of cases. Hardly a cause for spending more money on it.