For the xbox follow these guidlines...
F drive, fill it to the max! There will be no performance loss by doing this..
dont entirely fill up E because you need to have room for save games and thats where they go.
XY and Z, dont put anything in there, thats where the cache is.
C, its best just to not put much in here, besides you only get 500mb in here anyways...
Since the xbox uses different partitions for its cache there will be no performance loss unlike filling up a normal hard drive running windows.