RE: Tuned Pipe for Y.S. 45 SE
I haven't set up a YS45 in a bunch of years..... but either the 7.5 or 8.5 pipe will work well.
For speed, you probably would want the 7.5 size. Just because its physically smaller and a bit lighter... and you will be setting it shorter (The 8.5 has a longer front end)