I use clip-ons on my old road bike...
I do a club TT every 2 weeks with that bike and I really think it helps with me road racing in general. It's a good 20min threshold workout and works as a good 20min FTP test.
I have that bike setup with the saddle a bit more forward then my road bike, but still at the 5cm limit...I do some UCI TT's with that bike and they check in Ontario...
For this reason I think I'm working the same basic muscles and definitely gets me use to riding faster...