There are a few 70s magazine articles that dealt with exhaust comparisons on our 750. Some have found little to no improvement, some have shown a fairly modest but measurable improvement.
Many aftermarket exhaust manufacturers put performance gain low on their priority list, well after cost of manufacture and looks. Better performing vintage exhausts, such as Yoshimura, RC Engineering, Kerker, etc are still desirable for that reason. So performance gain potential is not only tied directly to the engine details you have, but the specific exhaust you are looking into buying. An aftermarket exhaust is very likely to be lighter in weight, never a bad thing.
For a modest over stock built engine, the performance improvement potential is probably fairly modest. It may feel faster (louder sounds tend to do that), and 1/4 mile times may come down a few tenths and pick up a mph or two. But it certainly won't unlock the hidden beast... the factory exhausts are not all that restrictive. The K0 pipes are actually quite good, and later baffle versions got progressively more restrictive. You could drill holes into the baffles on your current baffled exhaust and make it both less restrictive and a bit louder (stock exhaust restricted road racers have done this with success)
Like all such questions, the answers is generally a frustrating "Well it depends..."
George
