I've run a thin aluminum air dam underneath the trunk floor to the rear bumper for years, but one; I've never reached or breached the 150 MPH point, and two; I've never looked at my mph figures after doing such advancements.
Too much to explain, but I strongly believe the trunk to rear bumper is a huge subterranean parachute and when closed off rather easily, can make some eyebrows lift, not the car.
I've even thought about foils and small dams at perpendicular points of the chassis ie: Trans and torsion bar cross member, rear spring hanger members, all painted slippery gloss, etc.) where they could lend a hand in underbelly air management. This could/would be equivalent to lowering the car all together.
I was even thinking that swooped muffler designs would help a whole lot in street/strip applications.
Mufflers, talk about an underbelly 55' Chevy shoebox right in the air stream.