He is very good on what passes for grass these days, with heavier balls aiding him as well.
That is not the complete story, he is not good just because modern grass has become slower. Watch his movement on the surface, how he takes advantage of the sliced lefty serve that runs away from opponents, how he controls his service games. He is very natural on grasscourts, also his passing shots are the best of all time on the surface.
His only weakness on the surface is really his 1st serve return which is quite bad really, stats say his serve is as difficult to break as the likes of Becker and Ivanisevic. That is to say, faster grass should make his serve even better.
Federer loses to Nadal on grass because he cant deal with his serve, not because he gets broken alot.
I dont know how faster grass would make his serve easier to handle for a player with a onehanded backhand like Federer or Sampras.