RB20 head can't be that bad, my cefiro arrived with a rb20 head, east bear 1.8mm HG on a rb25 bottom end.. Was good enough to compete in the advan drift meeting.. Theres an r32 about in japan built by garage testorossa (sp) with an rb20 head on 30 bottom end.
Hands down would take a 24 if it had enough research and $$$ are behind it.