The blowing/frying of Fets is based off of the amp draw to them, not the voltage. The amp draw does increase as the voltage increases and under ideal conditions a stg1 or stg2 motor will be safe on stock elec. with lithium batteries. The problem you run into is less than ideal conditions.

Xmodsnuts, for example, is using a rather heavy battery set up. The heavier set up is going to add more strain to the motor, the more strain or load on the motor the higher the amp draw goes. This is where it becomes unsafe. Binding in the gears causes more strain in the motor, towing, pushing, trying to get unstuck or anything else that adds more load or strain will increase that draw from the "idle mark" for lack of a better way to say it.

So yes and no. If you use a weird/unorthodox set up like XMN then updating your FETs is probably a good idea. If you have a six cell then you are fine. If that makes sense.