As for the bottom end power, Maybe I'll have to wait and see if a GN400 turns up... Even though they put out 2bhp LESS than my 250 apparently, but I'd imagine torque in lower levels would be far superior
A mate had a GN400, as mentioned previously the 6V lighting wasn't very good. A few mods and it was pretty quick though.
If you're looking for a bigger single then its worth bearing in mind that the later singles have balancers which smooth things out a fair bit.