Try taking the glass canopy off and see what your temp swing is. My temp fluctuates between 80-81 with it off, and 79-84 when it was on with lights.
That's with 192wtt PC's, I'd hate to see the heat swing with MH.
I added eggcrate to the top cause I was scared my sixline might jump.
Advantage of a glass top: No jumpers and reduces evaporation(less topoffs).
Disadvantage of a glass top: Higher heat, less gas exchange, more cleaning.
The eggcrate alleviates my fear of my fish jumping so the only disadvantage is I have to topoff every 1-2 days.
Got the suggestions from these boards and I fel this has worked well with my setup. I may try a fan with the glass top on as an experiment later.