Sorry bad math on my part! It was a 7/16 drill bit.
7/16 in ~ 11.1125 mm
Yes the Mamba has great punch in the bottom and midrange, which is what I'm looking for in city driving and hill climbing, but I'm also hoping to squeeze 10 more mph out of the bike for the flats without sacrificing on hills too much.
From combing the forums, seems like people are usually able to hit 35-45 with the Mamba even thought it's a 'low/mid pipe'. Maybe that's only with a Polini up front.