Linc, aren't Hyper8 tires closer to 4.25"? That can make a substantial difference.
Also, what is the deal with such a high kv? Is the goal for a fast buggy? (I don't want to read through the whole thread - I'm lazy
) Besides, a pinion that small will see faster wear. Depending on exactly how fast you want to go, I would shoot for a slightly lower kv, something around 3500-4000 and use a larger pinion - something in the 14T range.