@Stede
Need a bit more info. So you set the 'fInitialDriveForce' high on Car A & then tested in game & the car was faster/had more acceleration/wheel spin etc. You then set 'fInitialDriveForce' back to normal on Car A & the power of the car stayed at the more powerful setting?
Car B you edited 'fInitialDriveForce' & nothing happened at all or it stayed modded when you tried to change it back?
Also, may I ask what settings you used? ie What did you set 'fInitialDriveForce' to?
Note: I really doubt you need to go above <fInitialDriveForce value="2.000000" /> as I've seen GTA V misbehave when settings are set way off the grid. Reducing 'fInitialDragCoeff' can help you increase top speed while keeping 'fInitialDriveForce' lower if you have a certain top speed in mind.

I would say it is worth checking your settings thoroughly for small typos too.