Maybe you'd see some increase in top end with something like a 30mm carb, but unless you are going to put in a big bore kit, I'd go no bigger than that. However, if you're going to put in the ig bore kit, stroker, etc. I'd go with the Keihin PWK35. It's a more modern design than the PJ34, and according to Wildcard, is a lot more reliable. Brandon from Wildcard calls the PJ34 "chinsey"
The stock boot that is on there now can go up to 28mm. That's it. You can, however, modify the boot for a bigger carb. I just bought the UPP 34/35mm boot from Vito's when I got my bigger carb. Then you know that it is going to be a perfect fit when you get the boot that's made for it.