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My CBR has an aftermarket carbon slip on thing and when I bought it the removable baffle was missing. It seemed obnoxiously loud at low speed in residential areas so I got the seller to post it to me and fitted it... It's now fine (and basically sounds the same) but I'm not sure if it's too quiet at speed. A quieter helmet might help but if I can't hear it properly at 60mph I'm not sure anyone in a car would.
My old blade's system isn't standard, it runs the same Yoshimura can that came on the bike when I got it, but I've replaced the rest with a stainless system which is the same spec as an original one. It looks nice and has a good "rort" without being stupidly loud. However, some of the sports bikes pipes I hear are really very noisy indeed, so I can see why people don't like them.