I'd posit that if the more laid-back windshield results in the windshield header being lower and further back over that open cockpit than otherwise, that it would help aero.
I sat in a Shelby convertible of some sort once at the Detroit Auto Show; I don't recall how laid back the windshield was but I do vividly recall noticing how close that windshield header was to my face. That was a very discomforting feeling- just imagine if I was a left-foot braker!
