I'm 6'2" 240lbs and I fit behind myself in my Stinger no problem. Head room is close, but not touching, and I have knee room to spare. Less room than the 5thGen LGT but that's no surprize. The center position is not a good place to hangout, but the two outboard seats are comfy.
I guess it depends how the driver likes to sit... knees up or legs straight.
Also, if you get in the back seat with the ignition off and the easy access drivers seat enabled, the drivers seat would be all the back on its rails. This can be reduced or disabled entirely if trash-compacting rear passengers is a concern [emoji849]
Getting in and out of the backseat is the real headache. Literally... your lucky if you don't wack your head, due to the low swooping roofline.
The G80 Sport is one half size bigger and same drivetrain. The Stinger is right in between the G70 and G80 in wheelbase length.