From what I saw, I notice Fed still owns fast courts indoors; mostly because he is the first one to go for the winner straight out, or put the opponent in a compromised position - the other top 3 don't do that as often.
I would argue that Federer hasn't remained at the top because of the slower courts, rather he is still at the top because he is just that good.
At the end of the day they are playing a simple game in physics terms. You swing a racquet to hit a ball. A great singer can sing anything, a great tennis player can play anywhere and be great.
K Factor K Surge