I heard that for hardtops there's a limit on windshield rake angle that yields no further drag reduction.
Somthing around 30 degrees from the horizontal if I'm not mistaken.
Do convertibles benefit from even more inclined windshields than their hardtop counterparts?
Something like 20 degrees from horizontal?
Of course you'll need a pretty transparent and non reflective glass, and a really really long and dark dashboard for such angles.
http://deepblue.lib.umich.edu/bitstr...pdf?sequence=1