Mylar can actually leak light. Solid foil doesn't.
There is no extra heat created. Energy in = energy out. Simple as that.
Hot spots are a myth. I've used aluminum foil. I've never experienced this 'phenomenon'.
Look on the halogen page. It claims a 80w CFL is equal to 500w halogen. Do the math with the PAReff. HID, as I recall, was 1.38 PAReff. CFL would be over 1.6 PAReff.
I don't think highly of using HPS solely. No disrespect to you, of course. Even incandescents in numbers will grow plants.