if you mean the real fancy $1500+ type mikuni carbs, they don't make any small enough for a cb750 let alone a 550.
Keihin makes the CR "special" carbs and a lot of guys spend big money running them but I really don't like their street manners at all, they're just slide carbs with no accel pump and they are like an on/off switch so their kind of aggravating to street ride on for me.
The PD's are your best bet.....if you really want mikuni's you'd have to measure the spacing and see if some older suzuki/kawasaki 4cylinder carbs would fit, something off maybe an early GS or KZ before they went to constant velocity carbs.....but all this just to have mikuni instead of keihin you're just setting yourself up for a ton of aggravation figuring out what jets and needles and needle clip position to run on carbs meant for a totally different bike.
Look into Steel Dragon for velocity stacks if that's the route you want to go. 77-78 cb550 would be what fits the PD carbs. Otherwise, UNI brand pod filters are the best pods.....but even better would be like a cognito airbox (looks like the old breadbox style performance filters but WAY updated) or fab something up similar. Wouldn't be too bad to fab one if you're any good with sheetmetal and a drill press....could even re-use the boots from the stock airbox. Actually, that brings up another possibility.....some people take off the actually filter housing from the stock airbox setup and just mount an adapter on the back of the 4-1 plastic intake plenum and toss a clamp-on cone type filter there.....seems to work quite well also.
