I've followed the what grip is Fed using threads a bit and studied Fed's game alot on slow motion video. In case anyone cares anymore, I wanted to show a video that is the most conclusive I've seen on his using a semi-Western.
My own view is Fed uses an Eastern to modified Eastern on return of serve and modified Eastern to near pure Semi-Western on forehand. Certainly on clay and to minimize his shanking he is now playing near pure semi-western almost exclusively the last couple of years. Maybe when he was young and had a more penetrating forehand he was closer to Eastern but he plays with more margin and spin now.
Pause the frames repeatedly at the 3:00 mark and you'll see there is no way anyone can say that's an Eastern or modified one. 2009 French