My main game is rFACTOR and this is the one that I more or less took as an example. However I also play "GRID" and "TEST DRIVE" and to be honest I dont find much difference between any of them regarding the engagement point of the clutch pedal. I will put it to you this way if the clutch in my car performde the same way I would have it in the garage to be replaced immediately.
A racing clutch is very different from a road car clutch; in most racecars the clutch is almost an ON/OFF switch. So, don't expect much slip from your G25.
Having said that, I understand the frustration in not having the bite point where you would like it. In rFactor, adjusting the axis "sensitivity" (this is really the axis linearity) should move the bite point. Are you sure you're adjusting the sensitivity setting for the correct controller number?