If the only thing you do is de-restrict your exhaust (combined with the high flow air filter you already have), you shouldn't need to go up a whole bunch of jet sizes. Probably one or two sizes larger than what you have now would be sufficient. Jets are cheap -- get them from treats. break>
If, after you de-restrict your baffle, you still want to go faster, the best bang for your buck (IMO) will be a performance exhaust. The Bullet, Boss, and Proma GP are all cheap, well-liked pipes, and there are numerous threads you can search for that debate the merits of one over the others (or you can just go for the Boss -- it's the cheapest, and no, cheapest does not necessarily mean worst). I'd say that you should get an extra 4 or 5 mph out of a pipe (might be more, might be less depending on how fast it goes already and how well you tune it). You should also plan on getting another two (or so) jet sizes up when you put the exhaust on.
There is a list of carb numbers and stock jets in the wiki (Bing article) that you can check, but if you want to be absolutely sure what jet you have, you have to take the carb apart and look at the number stamped in the jet. To know whether you have a 14 or 12mm carb, look at the 3 numbers stamped on the outside of the carb body. It'll be something like 1/12/432 or 1/14/553 -- the middle number is the carb size.