I don't think he would have done well on fast courts. He hasn't even reached final in the Cincinnati masters yet
I don't think he is good at adapting either. He never won WTF and he never was able to adapt to Madrid when they made changes to the clay.
Yet he made two SF and one final there, stopped by Federer each time.
He won Olympics, which was on fastcourt, beating Djokovic and Gonzalez in the last 2 rounds.
He won Madrid when it was indoors, coming down from 2 sets down to beat Ljubicic.
He made Paris indoors final.
He won USO.
He won Dubai beating Federer in the final.
As Strongo said, we will never know the real answer. While Nadal was always destined to become a monster on clay, the way he played the game in his teens does show signs of promise if HC were kept fast in a couple of locations. Nadal hit some absolutey brutal(flatter) forehands for his age and at times looked like a lefty Gonzalez, with all that power under the hood.
The problem with these kind of threads is that we always assume the players would play their current game(tailored to these conditions) in any kind of era, which I think is kinda bogus. Edberg, the prince of S&V(if not the king) said some time ago that he would rarely come to the net if he played today because it is easier to get passed nowadays and it's not worth the risk. Yup, THE Edberg would most likely be a baseliner in this era.