It's not just that it flows better, it's that it flows! Look inside the 20r manifold, it's just 4 holes dumping into an open chamber. Exhaust gases just come out and tumble around all willy nilly. Using a later model 22r manifold will be like using a small tri-y header, since it is a tri-y design. A header will help even more, but I'd look for a good used one if I were you. I got my nwor off ebay for $125ish used.
