Contrary to what some say, no engine wants "back pressure". What an engine wants, and in many cases what these people really mean, is the "ram tuning" or "pulse tuning" that is associated with relatively small diameter header pipes of the correct and relatively long length, which inherently result in back pressure. The back pressure, BY ITSELF, is a BAD thing, by reducing scavenging during the valve overlap period. But when correctly tuned it results in a strong negative-pressure pulse during valve overlap. A butterfly restrictor is just going to cause positive back pressure, without the side benefot of the following negative-pressure pulse.
The butterfly gizmo in the exhaust is a noise and emissions thing, although in some cases (some years of ZX6R) it's been used to deliberately de-tune the top-end power of the engine. (Why they would do this, is not known.) In normal cases, the butterfly valve is wide open at wide-open throttle, therefore it has no effect. It partially closes at part throttle (when power output doesn't matter anyway). Doing this reduces the amount of pressure pulses sent down the pipe to the muffler (reduces noise) and by increasing the back pressure a little bit, it CAN be used to keep a bit of exhaust in the cylinder at part load ... acts kinda like an EGR (exhaust gas recirc) system with minimal extra complexity. Only reason to want to do that, is emissions. I suppose it could be used to soften the power delivery a little bit, although the secondary intake throttles are mostly responsible for that.
