What's important here is not how good Diaz' boxing is, but rather how well he can use it. He could have the best boxing in the world, but if GSP uses his footwork to keep him at kicking range it's not going to do Diaz any good. Think of it this way, you could be a great submission artist like Demian Maia, but if you can't get the fight to the ground you're pretty much screwed.
And that's what makes this fight interesting. GSP has the tools to take away most of Diaz' boxing, by using speed, footwork, and kicks he can stay out of the kill zone while scoring strikes against Diaz. The question is will he use such a gameplan and can he stick it once Diaz starts doing what he's famous for. If he can, it's an easy win for GSP, if he can't or doesn't, all bets are off and Diaz gains the advantage.
This fight isn't about striking or grappling skills. It's about who can fight smarter, use a good gameplan, and have the intelligence & discipline to either make adjustments to the plan or stick with it when things don't go quite as expected.