Back in the a days these bikes were current, 4-1 were performance plays. There were shoot-outs in the magazines where they fitted CB750s with all the different pipes, did some minor jetting and ran them down the dragstrip. To the winner went the spoils (sales). RC Engineering was often the top pipe, and sales, with Jardine giving them a run. jardines suffered in the looks category and didn'tsell too well IIRC. After Jardine sold out and RC closed, Kerker bubbled up as the strongest pipe mostly on their reputation with the larger 1000cc SuperBike racing series, as the 750 class was losing popularity.
Yoshimura's pipe was perhaps the strongest, but they didn't offer a street legal version so it was often excluded from these tests.
Nowadays, it seems to be mostly a looks and convenience contest. Mac wins for economy, but I wonder about the performance. Seldom if ever tested. 2-1s are good for bikes that want throwover soft bags or hard bags. Ground clearance suffers, but judging from comments on this forum, that's the furthest from most peoples minds.
The new(er) CycleX 4-2-1 is interesting. Why go to the trouble if not to produce better power and clearance? Their's may be a good offering. I'd like to hear and see a test.
