"Andrew
The most significant feature of iGP that makes our cars "unrealistically" fast is KERS. That will give you a number of seconds advantage if you use it for an entire lap.
Why you realized the KERS as it is and not like it is in realitiy (limited Energy per lap)? If the ralization as a function of brake energy is to complex a fixed amount of energy (in seconds) per lap might be quite more realistic as the posibility to use KERS for one whole lap. (Sorry if I understand it wrong, as a lv 5 Manager I don'tuse it by myself atm. Hop to get in contact shortly ... :-) )