I'm running W7 Home Premium with an R9 290. I usually keep track of how my hardware responses to the things I use and I've noticed that getting into the game and alt+tabing back to my desktop still makes my GPU load to be the same as in game. Needless to say, this shouldn't happen and no other game is doing it.
Just fire up the game, even in the main menu, just alt+tab out of the game, you'll see the gpu is still working. Is there any aditional info (logs or whatever) I could provide?