Suzuki SET Valves
There is a lot of controversy over Suzuki SET Valves or exhaust valves. Because Suzuki say they are designed to create more torque at low rpm but many people say they are just used to get through noise emissions.
As far as I can remember if you place a restriction in the pipe then the gas will speed up past the restriction but then slow down again, so not achieving the velocity increase to scavenge the cylinders. See link for SET Valve removal
SET Valve Maps
These are the Suzuki maps for the GSX-1000 set valve (Suzuki GSX-S exhaust valve) . The figures in the table are the percentage the SET valve is open, 100 is completely open.