Okay folks, Mr. Consumer here. Remember the original question, which was basically is the lifespan of a 90+% really 2/3 that of an 80%. The reasons were longer run times and "design characteristics", and I suspect the latter include two-stage vs single-stage.
I was taken aback by comments that a two-stage is more for comfort than efficiency. Also took note of another statement that a variable-speed blower is more efficient that a single speed (and I would assume a multi-speed, too).
Let me boil my concerns into a simple question: What type of furnace would give the lowest overall operating cost/year, electrical & gas combined? At this point it looks like I would want a 90+%, single-stage, with a variable-speed blower.
BTW I may be an ignorant (but learning) consumer, but I'm not too bad at math. This argument on duct loss keeps comparing apples (input BTUs) to oranges (output BTUs). Please get past that (my ducts are in the conditioned basement, so my losses end up in the building anyway) and perhaps discuss some other efficiency/lifespan differences between one-speed single-stage 80% and variable-speed two-stage 90+% furnaces. That discussion might be helpful.
Sure hope I don't get this kind of cross-talk between the contractors that bid my system...