Like was already said, depends on what kind of riding you do, and other mods. Do you want low end, mid, or a top end shriek? It all depends on what you want, all the pipes listed will increase performance, some more in certain areas than others.
I second the Eric Gorr notion. I have met and talked to this guy and he is great! Really knows his stuff. Check out his website at Eric Gorr.com I'm sure there are alot of other good porters also.
It sounds like it may be getting too hot. I would check oil level, clutch fibers and adjustment. Don't forget to line up the marks on the case and clutch arm before you adjust the freeplay on the lever.
I am on my 2nd set of plugs since new, I have rebuilt it once when I had some motor work done and changed the plugs just because it was all new, never changed since. Oh, that was back in dec 2002! No reason to change if they still work eh?
Wayne National Forest is a good place to go and Haspin Acres in Indiana is nice, 2 moto tracks there. It's in Laurel, IN about 25mi West of the Ohio border almost straight weast of Indy. That is where the Midwest roostfest will likely be.
T4's? Where did you find them? I would start at a 310 main, needle in middle, and if I remember right T4's like alot of pilot so a 30 will probably do. Later
They are very similar in power bands, the Pro circuit is a copy of the T5. Go with wich ever is cheaper if money is an issue or what you like best if not.