cb550 k had 4 into 4 system and almost identical preformance to the f models in fact marginally faster in one mira test, what baffling etc you use will obviously infuence this, a feiend of mine had a massey tractor silencer, (unbaffled) and homemade cruddy airbox and he reconed the bike (550 f1) was faster than my bike (550 k3) with its motad neta so possibly less baffling helps (k3 also used leaner jetting apparently), my 500/4 was also a bit quicker with its Very loud triumph 'silencer' caution though I've heard short streight thughs can apparently lead to burnt exhast valve seats not to mention upset the old bill. I'm afraid I can't help with sourcing pipes directly but try apincorporated as they may have something in.