Yes, you will gain some speed, but not a really noticeable amount. You would probablly gain about 7 f.p.s. which isn't going to be noticeable! I would say that you just keep the 125 grain heads and the set-up as it is, as long as it is comfortable for you. The heavier arrow will carry more energy down-range than a lighter arrow, plus the 125 head will up your F.O.C. meeting which is usually low if you use a 100 grain head on carbons!!!!!