What kind of plastic is that you used to line the fish room? Looks like a ton of work, not to mention expensive, but if it allows you to keep the tank in a 100% humidity room, at tank temp, it should save you a lot of evaporative cooling.
How are you planning on plumbing the sunken down area? I don't imagine you gravel vac the whole tank, so I'm assuming you will run rugf or something there? Might be useful to stick a pool porthole light or something down in that area in case one of your fish is sick or injured and hiding out down there, so you can see/diagnose/net the bugger.
Is there a floor drain under the tank? That would be a nice 'peace of mind' thing to have, although if there isn't one now I guess you wouldn't be adding one!. How much did it cost to heat and filter the 800? Do you think you'll get some power savings from the humidity and insulation improvements?