I wouldn't upsize the diameter unless you are sure it can handle it. Larger tires always look better, but unless your trans has been geared for it, you might find the engine struggling/bogging down on the hills, as this changes your "final gear ratio", so to speak. I had too tall of a ratio when I first started in the buggy scene, and It was a drag until I got it corrected. I ended up with 13.00 tires running a similar engine setup, putting out between 125-140 horses. It limited me to only 60 mph, but that's fast in the sand, and it allowed me to pull the same lines/ hills as my buddies with the v8 cars, because of the low final ratio.
I hope this helps; congrats, and have fun with the new toy.