Its a shame PD make you need to buy the DF Pro to fully appreciate their game.
But it really does change the whole experience. I don't know if the DFP affects the gas, but the assists on the steering may be making tiny changes in the steering to stop you spinning out from flooring it too early.
Take my word for it, you have to be soooo careful with the M5 on street tyres with the DFP, gently bringing in the gas as you straighten the wheels out. The slightest bit of turn on the wheel as you gas it and...
After trying to get a half decent lap with the M5 on strong feedback and street tyres no TCS, I was ready to kill!
Good luck with the Z-tune, the top times are high 1:38's. I was pleased with mine because with the DFP, I swear that was hard work