You also haven't said anything about testing your engine for back pressure to see if you really do have back pressure or what it is, that way if you do get a different exhaust you can again test the back pressure and see if it has changed.
The formulas that tend to be published in books on exhaust design tend to also be tested on real engines as well, they are also the same formulas that lead people to putting 2.5" exhaust pipes on their vehicles that they are using for racing with a 4 litter engine!
But the bottom line is, you want to keep your exhaust moving at around 240 feet per second and a larger exhaust pipe is not going to allow that to happen unless you have a much larger volume of exhaust as well, that larger pipe will end up with a much slower charge of exhaust moving down it so I have to wonder what good does that slower exhaust give you?
I really wish I could find an online calculator that did the math for you that allowed you to put in the engine size, RPM and get an ideal pipe size but Summit Racing has a crude version that you have to have to use a pen and paper to do your math.
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos