you can "overcarb" an engine, by fitting carbs with throats larger than required. granted you could fit fatter needles to get the fuelling down but I assume that larger throats will cause problems.
people in the tuning world often used to fit larger ones for "pub" horsepower. when I was into escorts, folk used to fit a set of twin 40's to a 1300 xflow when a single twinchoke 32 DGV would give just the same horsepower, as the engine's air and fuel requirements were more than enough from the sigle carb. Bet it didnt sound as good though!
A classic example is back in the 60's some cars (rovers I think) would be fitted with a pair of 2" su's, but the float would be restricted in its travel to prevent overcarbing- so they fitted twin 2"ers to make the spec look good, whereas a pair of 1.75" su's would have done the job equally well.
I'm not that up on bike carbs, but when it comes to throttle bodies (fuel injection) there is a general formula which can be used to calculate what size you need, I would assume there is a general rule of thumb for carbs too.
I have some fireblade (CBR900 929 fuel injection) throttle bodies, and they are good for about 200 BHP with 40mm chokes, I would imagine the carb'd 'blade would have had similar sized throats - would assume they would be far too large for even a well tuned MA12- but saying that speedle has a lovely pair (ooerr) of 36 DCNF's bolted on, any they run great- but we don't know what size throats are fitted to those.
As the CBR600 units have been used by members of this forum, then I say go with those- as their experience of fitting and using them will outweigh any extra cost than fitting a different type because you can "get them cheap." I picked some goofed ones up for less than a tenner a while back (need rebuilding tho) so if you hang in there you could get em cheaper.
HTH, just my 2p, feel free to ignore!
