I am definitely significantly slower with my G25 than I was with my DFP.
I used to use 500-720 deg on the DFP and, if anything, the G25 wheel is EASIER to use in 720 than the DFP in 500 because the FFB is so much better. So I don't think the wheel turn is slowing you down.
Going from left-foot braking to right-foot braking slows you down considerably, as does having to heel-toe downshift rather than just 'blip' with your right foot.
Gone is your ability to carry a little throttle under brakes (helps with stability without hurting turn-in; sort of a sneaky way to get 'the best of both worlds') and your transition from brake to throttle and vice-versa will inevitably be slower and not as smooth.
I have been a hardcore left-foot braker since GPL back in the day, and in every sim I have played I've always found it beneficial to carry some throttle under brakes... not anymore, I'm afraid!
But, as others have mentioned, it is now way more fun and way more 'realistic'! It would be great if someone setup a hardcore G25 server so that we could all compete against each other.
With regards to setups, be careful with the WR setups as some are designed to be used with a trailing throttle under brakes. Just move the brake bias forward 2-5%, and that should fix any 'spinning-out-under-brakes-if-I-even-THINK-about-turning-in' issues that you (I!) may have.
Viva la G25!