I have had some first hand experience with (reasonably) large, high quality optics (Society's 20" made by late Bill James). There are few problems that were not so obvious at first instance. One is LOCAL seeing. A large telescope has enough mass to change local atmospheric pattern. In this instance, a large thick mirror, combined with a massive supporting metallic structure takes a LONG time to radiate. If temperture keps falling (unfortunately most of the nights that WILL happen), there will be NO equilibrium, and although temeperature gradient might not be big enough to ruin mirror's figure, it is more than enough to create such a turbulence inside the tube that my nearby 8" NEVER fails to beat the behemoth on planets.
But, this is not a simple issue either. Few weeks ago I've had a chance to look thru Barry Adcock's "underground" telescope (a 8" f/30 stationary refractor with a 12" zerodur siderostat mirror and another 8" zerodur flat that folds the light path under the ground into nearby house's cellar). Closed tube, perfect equlibrium (VERY stable temperature underground), yet his nearby 12" Schiefspiegler was offering substantially better images (details easily visible _within_ the Red Spot), not all the time of course, but when seeing settled. The aperture _did_ win in the end. It was much more comfortable, of course, (and warmer) inside the house ... The only worrying thing is there is no rain detectors !
Bratislav