Grinding the bushing will give you a small top-end boost as well as some low-end, as the variator can close farther.
There were other restrictions than just the variator, but I am not sure what specifically was the difference between the 25 and 30mph versions. If you get an exhaust, carb, variator (or mod the stock one) and intake I would expect you to be as fast as a 30mph bike with the same mods. To the best of my knowledge there was only one cylinder for all 3 speeds, so that shouldn't be an issue.