Well with his superior serve and volley back him up with his average groundies, but if he didnt have those two weapons he would just be an average top 100, although he has that amazing running forehand, i honestly dont know how he pulls that off with an eastern grip. dont get me wrong here though im not saying eastern is bad for everybody who plays tennis, i was just saying that at pro level beucase they hit with so much force, its not an ideal grip to use since it doesnt give you lot of margin for error and power. But if you are playing at level 4.0 and below eastern is not a bad grip to use it at all. I hardly see any juniors who plays competively uses eastern i guess its because their coaches didnt want them to develop a technique which they are not going to use in the future. The coaches who truly understands the modern game of tennis, i can gurantee you they are not going to teach their players to use eastern.
And for Federer he does not use eastern its so obvious he uses semi-western look at how he holds his racquet and how he swings. Federer uses his wrist so much that its almost hard to imagine that he is using an eastern.