in a race, if you lower your bhp to 800 (for example) you will be underpowered and should not be able to keep up with the ai.
the opposite (rising your bhp to f.e. 900) is not necessarily true because of the rev limit (in physics and torque).
the behavior you describe in your post looks to me as if the ai is having a differentiation over the perf file, but the player car goes just over the physics, but as i said before, i don't have any experience with hotseat-mode!
anyway, if my assumption is right, then by editing the physics file, you should still get same laptimes for all of your (player) cars, but different from the ones before. i'm not quite sure, how this will affect the ai!
My personal GP4 files at : www.4shared.com ______ My Blog at : Gitanes GP4 work
* Performance Files * LoadingScreens GoogleMaps * GP4 Tracks and Cars Databases