Yeah you do need to give the pro8200 plenty of breathing room, especially on sides, it gets hot.
As to screen size, 80 inches should work, and even bigger. Comparing it to hdtv 80 inch which costs about 4,000 dollars. I get 100 inches or more no problems. Also, the pro8200 would look better than hc4000 at bigger sizes (brighter).
A couple of questions to ask yourself.
Do you want a PJ that will mostly stay in this one room or one that you could move with you, to bedroom, den, porch, friends house, kids room? Do you want to lock it down and throw away the key, or stay flexible, mostly in main room, but pack up and carry when needed. Because the pro8200 would knock the pants off the hc4000 in Flexible placement in many different type rooms, it has MORE lumens for living room and outside, and has more inputs/outputs, and 20 watt speakers that you could add a portable subwoofer for deeper sound. With the hc4000 I would have to carry sound system with me, the pro8200 has built in sound system in a pinch and in many rooms the hc4000 would just be too dim.
Do you want 400 dollars in your pocket or is 1,127 worth it for tiny bit of blacker black at expensive of dimmer picture? Coderguy sold has hc4000 because RBE bothered him so much. And if he then bought the pro8200 and he experiences NO problems with RBE , it means you don't have to worry if you or your wife and kids are sensitive to RBE with this PJ.
Now the downside with buying from newegg is NO return policy, they just exchange it if it goes bad, Amazon has 30 day return policy. So , that is something to take into consideration.
With the Pro8200 you are buying a cross-over PJ, meaning you could do business presentations on it, with mic input. Or use in school, or take to some meeting along with portable screen. The Mit hc4000 is lock into place and throw away the key in BATCAVE. Coderguy is Batcave guy, I'm into portability with option to lock down into batcave , so the little bit of less blacker blacks doesn't bother me for the other gains I get from the pro8200.