I've modded several exhausts, with varying results;
Honda CL450, the spot welds on the scrambler pipe were cracking, bike ran terrible and sounded awful. Had a friend drop a tig weld on each dot and the bike ran great, but had no sound.
Kawi Vulcan 1500, 2 into 2, hole-sawed the end cap out, sounded great and seemed to run better.
Honda 750, 4 into 2, removed and cut off two inches of the snooters, sounded really good, loved it.
Another Honda 750, 4 into 1, had a big Yoshimura can on it, way too loud. Re-wrapped it four times, never did get it right.
Honda CB1100, 4 into 1, drilled the end plate so much there was almost no end plate left. Barely made a difference. Still have the bike, tho.
My NTM Yamaha V-Star 1300 came with a Cobra pipe. Sounds pretty good.