I'm a little surprised you haven't managed to recoup more than that but of course I don't know how large your entire game folder is. Did you get rid of any extra copies of your game folder on C and empty the Recycle Bin? Have you tried rebooting since freeing up the space?
From what I could see on your dxdiag, both drives are internal. There shouldn't be any noticeable difference in read/write time between the two. The D drive on a setup like this isn't really your backup drive, it's where usable data not directly related to Windows and programs are meant to go. A backup drive would more typically be an external one. But the very worst case is that you could always put things back the way they were before (wouldn't really recommend that though).
