The MOSFETs won't be damaged as-is but they do get mighty warm. I ran mine stock for quite a long time but I recently paralleled the drive MOSFETs on my spare PCB just to see if keeping them a little cooler will help at all with fading. Haven't tried it out yet though as I've been busy working on parts for a couple guys and testing the air cooling setup. I've also figured out a way to get a little bigger fan in on the side of the wheel opposite the motors, which might help some too.
I really wish I could get into the firmware a bit but I'm not real up on that stuff. I wasn't really posting a lot during the time I experimented with the following so I forgot to put this info up - a conversation with mrbasher the other day reminded me to put it up here when he mentioned using it as a possible approach to help keep things cooler. Anyways, here you go: I did some playing around with Racer XX's info regarding the temp sensor quite some time ago - which is a thermistor - and discovered that the controller limits the pulse width to the motors to 88% (normally it allows 100% pulse width) when the motors reach about 140°F. This would have been awesome if it was an infinite range - but instead it's just that one cut. You either have 100% pulse width, 88% pulse width, or 0. If the motors should reach 150°-160°F, the controller cuts all power to the motors until they cool. So anyway it could be beneficial to use that with the Bühler motors to enhance reliability and almost eliminate heat fade with the stock cooling but they won't be nearly as strong. They might well run with a stock power supply though.