Well for the stock jugs they make pistons that are .100 over bore. that is the largest I have seen. Then there is a big bore kit in sizes including 392, 421, 443 and 465. They dont stop there. it is all in your riding style and experience. A brand new off the show room floor banshee has crap for suspension.
If you are just looking for more power than look into pipes, cool head, timing plate and maybe a port job.