fmf fatty? I think phatty is reserved for other circles. By converting to 220 do you mean installing a 220 jug and piston on the 200? I know of a 225 and 240 BB kit for the 200, but no 220 kit aside from using kdx220 parts.
In general, the 200 is a bit faster, while the 220 has more on the low end and doesn't rev out as much. I would expect more of the same from the big bore kits. My vote would be to freshen up the top end on your 200. Check the condition of the cylinder, piston, rings, reeds, and then work on jetting. The 200 should be plenty quick.