Read this thread on modifying the AI. The thread is from GTR2 (a predecessor to GTR Evolution) but pretty much all of the items in this AI tutorial carry over to GTR Evolution including your desire to modify AI qualifying and racing lap times and personally I believe one of the more important mods to make is to modify the various car DMG files in your race increasing the "AeroMin" value if you run races that take more than 30 minutes to complete. Anyway read the AI tutorial and apply most if not all of the recommendations and your AI will be better racers.
As far as pit stops, I have seen mixed results. In some of my longer races I have had great tire/fuel pits stops where there is not much difference between myself and the AI (I did a 42 lap race at the Lemans 77 track and every car made 3 tire/fuel pit stops). But I have also had less than desirable results with shorter races and yes it pit crews appear to be slower. But turning off their ability to fix damage to your car will speed them up. That is assuming you make it to the pit stop with an undamaged car.