Got a theory that I'm going to try - probably a load of bollox, but you never know.
I'll dig out the gamepad and use that instead of the G25 on a circuit I know quite well. I'm thinking that over the course of a whole lap, the all-on acceleration and all-on braking with the buttons has a bigger impact on the lap-times than the gradual delivery on the G25 wheels. If that theory holds true, you're later on the pad with 100% braking than the G25 and are 100% on the throttle coming out of the corner, rather than feeding the power in gradually, with the pedal.
I'll give it a go anyway - see what happens.
D.