Yup, most MOSFETs will have no trouble running at 125C! The power supplies in some of our products occasionally run at 180C, without heat-sinking. But these aren't designed to last as long as a motherboard VRM.
In general cooler is better. At high temps you'll see temperature related aging accelerate (e.g. solder, PCB substrate, electromigration in the parts), but you'd probably be okay. The PC would become obsolete way before the VRM fails. The worst case I can imagine is the MOSFETs and inductors heat up, bringing the local temperature up to a point where the controller IC enters over-temperature shutdown, but it's unlikely.
Personally, if the current heatsink is too big I would just find smaller ones and attach them. I don't really have a good reason, I just like parts running cool :)
In general cooler is better. At high temps you'll see temperature related aging accelerate (e.g. solder, PCB substrate, electromigration in the parts), but you'd probably be okay. The PC would become obsolete way before the VRM fails. The worst case I can imagine is the MOSFETs and inductors heat up, bringing the local temperature up to a point where the controller IC enters over-temperature shutdown, but it's unlikely.
Personally, if the current heatsink is too big I would just find smaller ones and attach them. I don't really have a good reason, I just like parts running cool :)