The shape of my '75 olds starfire was essentialy the same as the Crossfire coupe. The rear hatch weighed about 100 lbs and was a real problem to lift without the gas piston assists.
I can tell you that at 60 MPH it would lift if the latch wasn't closed properly.
At 200,000 miles on the clock, the latch was relatively inoperative and the gas cylinders were useless. Tuck tape was tacky so I used a bungie chord.

So, I am used to a big flap going up and down in my rearview.

I don't know about the coupe but the vert gets dicey without the spoiler at about 90 MPH. It's not that the car is uncontrollable, you just lose the hard edge.
The lift is noticeble.

roadster with a stick