There is a whole discussion of this on the other forum. It starts with a post by tprice.
http://www.rcgroups.com/forums/showthread.php?t=1592741&page=13
klkirkman is an accomplished naval architect that designed RC boats back in the 70s or 80s.
The gist of it was that putting the keel too far back made the boat much too difficult to tack, more than offsetting any advantage downwind.
I drew My last IOM 7 years ago with the exact purpose to find out what would happen. so I took my second last IOM and kept everything where it was. the leading edge of the fin is at 500. and my last IOM the leading edge was at 550. and from what I could tell. the effect was better. I could turn the boat better. and going downwind, I did not have a problem with control. the only trouble I seem to have had was in a gust. the boat would pitch-pole in a heavy gust. that’s it. I have drawn an IOM for a friend and put the fin at 550. and he has not complained other than when I gave him the boat. he was sure the boat was going to be bad. but it wins his club championship every year for him.