I am using a default installation of the game. i'm in DC's mclaren with the default HP on the engine. I've found with GP4 more than any other GPx game that even when you think your going fast and pretty near the limit there is still loads of time available. we had a comp like this at the nurburgring a while ago, I can't remember the times exactly but my PB from before we had the competition was something like 4.5 seconds slower than my PB after. there's just so much grip available if you know where to find it.
Using a keyboard certainly slows you down though becuase its a digital controller. the steering help makes the disadvantage there pretty much nothing but in terms of accelerating and braking it really costs you a lot of time in corners like copse, maggots and becketts.