I have tried a 48t and 50t rear sprocket. They would be nice for a stock bike but if you have any mods, the stock gearing is great (I belive that Jeff says something about this on his site). 2nd, 3rd, and 4th gears (while on the pipe) will keep you at good race pace with the stock gearing. With the larger rear sprockets I felt like I was riding slower on the straights, because the bike reved out quicker and I backed off; and I didnt seem to carry enough cornering speed since I didn't have to keep the revs up and use the clutch as much in order to feel the "drive" after the apex. Taller gearing seems to keep me faster, since I try to keep it on the pipe as much as possible. I don't know about going smaller on the front (countershaft) sprocket. I have only heard of people going higher, to a 14t front, and I know only a few who have done this. Most of the pro wrenches and techs that I have talked to recomend going only 1 or 2 teeth in either direction when choosing a different sized rear sprocket. Going from a 13t to a 12t front sprocket would be the equivelant of ~ a 3 or 4 tooth smaller rear. I suspect that a change in gearing such as this, for a 220, would create a more mild low-end that pulles a little longer and a mid-top that pulls much softer (almost flat) but a bit longer. I think that a 220 with stock gearing and a rev pipe would work better, and a 200 with a tourque pipe would be even better. If youre looking for a softer bottom and more top, a 200 with a rev pipe and porting will pull just a little less than stock on the bottom, then explode like a big bore 125 mid-top.
-Nick