I also have a fusion drive and my windows is home premium so it only uses 16GB ram.
Battlefield ran great the first 2 weeks but now it stutters like mad. I may try and un-install and re-install it and see if that works.
The specs on the imac runs the game really smooth, but there is just something going on with my set-up.
I'll play around this weekend and write back if I fix it.