I have similar gpu in my rig (I7 6700, GTX 970 4gig, 16gig RAM) with pretty high settings and get the same 20fps and stuttery in that Caucasus cold start mission but with SC and 6-8 statics in a fast mission I didn't see less than 30fps and it was playable. Im surprised you're only getting 5fps more with empty carrier. Not that 30fps is much better but at least you don't feel performance drop much at 30fps.