My opinion on this, is that it could make a difference, although very small. The advantage of having the game data on another drive is that the OS background process's read/writes will take place on another physical drive and thus don't interfere with the game reading/writing. I wonder though if you would notice this benefit while actually playing the game, since most of it should already be loaded into the RAM.

I'd say it's not worth it if your drive is large enough and you defragment regularly, but maybe some other members with real life experience can comment on it.

If you have two drives I would recommend if you ran running a RAID array, will boost performance majorly :)