Fat Boy? :) Maybe a Fatty? By FmF's description, the fatty was an 'all round' pipe that improved power 'across the entire band'.
It wouldn't seem too likely that a 240 sleeved 220 would need a whole lot of help on the bottom end. If you're still running the stock 33mm carb that comes on later 220s, the bore to 36mm that is part of the RB mod will be a big wakeup!!
BTW..try the 'search' function on this board. There has been MANY threads concerning the RB mod.