[Rez]

A Gen1 with two Li batteries does not need a FET upgrade, unless it also has a hot motor. The batteries don't matter to the FETs as long as you're not exceeding the rated voltage.

If you exceed that voltage you can blow the whole car, not just the FETs for the drive H bridge, but the radio Rx, the steering, everything.

2x2 FETs can not take twice the voltage, they can take twice the current, and the amount of current depends on the load, in this case, the motor. NOT the batteries.

Where did you get the idea that 7 cells wasn't enough power for the weight? If each cell delivers the same amount of power and weighs the same, how can an 8th correct this? I would be more concerned about the voltage. Seven cells at 1.5v each is 10.5v, which is dangerous to the health of your car. You might get away with it for a while, but you're pushing your luck.