Tracy Saylor RV-6
Tracy has a very fast RV-6 on the west cost. He tried the big swoopy wing body fairing and found it did nothing. He went back to the flat metal cuff and rubber seal.
The original RV-4 had the LE cuff and than sheet metal fillet the rest of the way. This is verses the current flat metal filler and rubber seal. I had the old style on my RV-4 and switched to the new way of closing the wing / body gap. I prefer the latter. It is cleaner looking. As far as speed I never really measured it because I made many changes and did not keep track of it, but doubt it made much differnce by it self, although my plane went faster with the flat cuff, but that was no doubt from other changes made at the same time.
I found that the LE cuff ends up having details like where it meets the side of body and is draggy. For some reason the fillet fairings are just not that effective. Sam James sells a whole fiberglass side of body set up. I guess if you put it on and used body filler it can't hurt. However from a building stand point it is more work, more holes in the body and weight.
Dave Anders with the fast RV-4 just has the flat fairing and rubber seal, no curved cuff.
I think it is just cleaner and lighter to go the way Van does it now.
George