Meat-head wrote: So is this a complete waste of time?
The centre pipe is about 2-1/2” diameter.
Everything I’ve found about a GPZ750 says it would be about 2” or 51 mm.
That depends on a lot of things.
Here are my opinions. Keep in mind what you paid for them.
If it does not fit, do not use it.
If it does fit and performance is not a priority and you are pretty sure you can get a mid-pipe and muffler to fit and you like it, use it.
There is much more to a performance exhaust than looks. The relationship between primary pipe diameter and length is very important to assuring the best scavaging in the fat part of the power band. The diameter and length of the collector is also very important.
A bunch of years ago I ran across an old magazine article comparing a bunch of aftermarket exhausts on a stock KZ1000. One of the systems was a very expensive Yoshimura racing system. It was clearly designed for modified engines operating at high rpm. On the stock engine it made less power in the mid range than the stock exhaust system did!