Jim, I hope I read your question right (tired here). I would say that if you didnt change the draw length on your bow when you added the string loop you wont lose any FPS but your anchor point is off now hence your draw too long. I shorten my draw by a half an inch using the loop so in that case, yes you will lose a few fps. But this could pay off in better accuracy, better groups, etc. I have weighed my loop and a brass nock and there is like 1 - 2 grain difference if memory serves me right. I really dont think my shooting improves either, the loop just plain saves my serving!
I would say if your shooting has improved, that is more important than losing a few fps IMO. Good shootin

*Become a better shot —> practice, don't just shoot*