The 100.5mm (20 over) piston is a good choice, you'll have to get the cylinder bored but you'll also know you are starting with a good round bore. If you use Wiseco pistons, that still leaves room for 2 more bores on the stock cylinder (101 &102mm)
what is the stock compression ratio? can you get the 11:1 piston in a kit with the rings, gaskets, and the whole 9 yards???? if so, where? if not, where is a good place to get the 11:1 piston. i may be tearing apart my top end to do some repairs and figured "since im in there, i might as well do some work". so after reading this thread, it kind of convinced me to keep the stock bore and just get a higher compression ratio piston and cam. but i do want to run on pump gas (91 oct. or lower).
Once again, I would go with the 100.5mm and not the stock bore size. If your machine has some time on the motor, the bore will show wear, get it bored 020 over once you get the 100.5 piston and you'll know it is back to factory specs. You'll get a much better ring seal and life span from a fresh bore.
I can get you the piston and top end gasket set, PM me if interested.
if you dont like problems at all go with stock. But i have been very happy with my 686 11:1 . stuck with a stage one cam so i didnt have to mess with the head now it runs a 14t front 39t rear sprocket and screams. so much funner than stock!