I've used the 15T for a month now, and am happy with it.
There have been a few instances where I've needed to change down to get decent acceleration, which never happened with the 14T
I think the bike would pull a 16T, but it might be a little to much, and the speedo / mile counter would be that much further out again
The only other thing I've noticed is increased chain noise. The original sprocket, has what i think is a rubber damper built into it, which the replacement hasn't