CPU cooling has become an art form. When I was in my teens a heatsink was normally a simple lump of black anodised aluminium with a row of fins. In this age of powerful PC processors there has been a need for efficient, yet compact heat sinks. Some are fan assisted, others passive.
If someone has gone to the trouble of designing an efficient compact heatsink for mass production (lower price) why not repurpose it for other cooling applications?
I applaud your thinking and see no issues with drilling the heatsink for attachment of the MOSFET's. Use some decent thermal transfer paste as well.
There is of course one fly in the ointment....... Some PC heatsinks are not as good as they claim. Use some common sense when selecting one. As a simplistic rule,a good heatsink needs to either be large and cooled by passive air flow, or smaller and air movement across it maintained by a fan (or a combination of size and cooling techniques) An efficient small heatsink needs a combination of large surface area and air moving over it to draw away the thermal energy. If you find a heatsink with plenty of surface area and fan forced air across such, it should shift heat well.
As I said, some PC heatsinks are a work of art these days....... Many thin fins around an aluminium core and a decent fan pushing air over the fins to shift the thermal energy. The GPU heatsinks can also be very neat and efficient.
Go for it.... Repurpose a PC heatsink
Fraser