With the newest version of GPxPatch - 4.44 - the easiest way to do this is:
1) Use 'Power and Grip' instead of 'Old Behaviour'
2) In TSM's Magic Data screen, make the power and grip section look like this:
A setting of '500' should mean equal to the performance file you are using, but some of us have found that with Power at '500' the AI are too fast in a straight line.
'495' for power has, in my experience, meant equal straight line speed for AI and player, assuming they use the same gear ratios and wings, and that the cars have the same power.
F1 Graphics Swapper ---
Lensdump - Renders and Screenshots ---
GP4 Performance Generator