My CB750 with 836 ported head, cam VM29 carbs ran really bad with a too restrictive exhaust system.
Worse than stock.
I used a silent baffle in my then new 4-1 (Alfa, delivered with 2 baffles, silent and loud)
I did not test before the long touring going south to Jugoslavia as it was called back then. Bike could maximum reach 130kmh in silent mode. Before 200kmh + squeeze it a little further to 210kmh according to speedo.
Engine sounded bad too, as it should explode
That was not possible in Germany on Autobahn. Had to take it out and riding really ear ringing loud!! Cruise speed mostly around 160-170kmh
That very silent system should probably work better on a stock bike.
A biker friend that had a Z1000 1977 with more power, RC pistons, headwork, RC cams, Dellorto carbs and reinforcements had seen same thing with stock pipes. Too restrictive is working worse when engine is modified.
Drilling good looking stock pipes will probably not help much.
I drilled my Lotus Root baffles/diffusers. 5-6mm holes in a ring in the bottom of them to help the flow with the 1005cc build.
Possible to undo if needed. Tap and screws or weld.
Taking them out completely too loud in the long run when cruising around.
Same bike used Yamiya No numbers before. Ran rather OK with no baffles.
Hindle 4-2-1 work better when open the throttles. More silent in legal speeds.
For a future back to stock to harvest $$$$$ or get money back, good to have the stock bits on the shelf.