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not much .... thats why they make the air cooled reflectors... cuz once the glass warms up... its radiating more heat as light is converted to heat... thats the greenhouse effect partially.. the other part is glass reflectcs IR (heat) and allows light to pass throguh... so light comes in... converts to heat... and cant get out as well as it got in.. 

Here's the kicker though, Lowe's and HD carry 400W HPS bulbs!
Buy one, test it, if it doesn't fire up.. bring it back to the store. If it does, well.. that's up to you. I'd recommend a mixed spectrum HPS bulb with autos though.