Glass is awesome. I was undecided at first. I never laid eyes on a mustang with this option. Just to answer headroom issue. I am 6'5" and headroom is no problem with glass roof. My biggest issue may be if i have to wear a helmet. I think as mentioned before it gives more headroom. As far as heat. All black with glass and doesn't seem to be any worse in the hot ass GA sun.
At 6'3" I was able to fit with my helmet (HJC Full-Face Modular Motorcycle Helmet) and still had an inch or two above my helmet.