The stats:

2012 G37x Coupe, Eibach lowered, SPC camber kit in the rear, OEM on the front. Miro 989 18" square set on Conti DWS 255/45.

Recently my car has developed a buffeting, for lack of better word, at any speed over 40 mph. It tracks straight, but the back sways, buffets...it is not at acceleration. Anyone has experienced something like it?

My wife, the joker she is, does a James May imitation when it happens... "buffeting... buffeting" (you gotta watch the old British Top Gear to get it).

Couldn't resist:

 

No idea what that means
Add​​​​​​ sways/ thread

 

Check your alignment , your rear toe could be off.

 

+1 to the alignment. When I had too much camber/toe taken out of my rear wheels my car would "shimmy" side to side over bumps.