For drag racing, make it a sequential "dog box" and you'd rule super street. IMO you'd make up for any lost ET of that extra shift by better splitting top gears up into the 'sweet spot segements' of the torque range.
A few years ago, the top pro stock bike guys all added a 6th (I think) gear.
Another interesting note: he NHRA pro stock boys have 2 or more 'half built' transmissions in the trailer during qualifying and on race day. After a pass, they down load the data logger, look at their notes, and make the final gear cog selection, and button it up. Since the trans is out between every round for clutch servicing anyway, they just hand the 'optimized' trans to the crew member to install before the next round
