The XV80 is a 2-stage unit with a variable speed blower, and SHOULD be running cooler/longer than a typical bang-bang gas furnace. Most 2-stage units (I'm not sure about the exact specifics on the XV80 controls) will step up both the flame and the blower speed if the thermostat isn't satisfied in something like 12-15 minutes, but always starts up in low-fire low blower speed mode. Does your air flow seem to speed up after it's been running for 15+ minutes?
The ductwork should be both mastic-sealed on the seams & joints (or FSK- taped with 2" aluminum tape, though that isn't as reliable) AND insulated to at least R6 (inside of conditioned/semi-conditioned space), or R8+ if the ducts run in a vented crawlspace or attic. Clearly it's more important to insulate the supply ducts than the returns, but both need to be sealed. In conditioned space you can skip insulating the returns though.
Thre are about 10 different models with different high/low fire BTU output numbers, which will make quite a difference in how long it takes to recover from a 5-degree setback. But 30-40 minutes when there's a 50% heat load isn't unreasonably long- it may be on the short side if the unit is perfectly "right-sized" for your coldest-temp conditions. But recovery time from setback is often a better measure of the thermal mass of the house than anything. Read off the whole model number (probably TUD********* ) on the name plate, or find the BTU input numbers on a plate somewhere- it's in there. Then with your zip code and description of the house's window size/type and insulation levels it's possible to take a WAG at what your design condition heat load would be.
Exit temps at the registers at high-fire high blower speed should always be above 100F (body temp), but at low-fire & low blower speed tepid temps in the 90s are usually acceptable, since there's less induced "wind chill" from the air flow. Most ~80-85% AFUE furnaces will have output air temps in excess of 125F right at the furnace's heat exchanger at high fire, with 110F+ at the registers if the duct design & implementation was decent. At low fire they're milking it for highest efficiency (==lowest exhaust stack temp), but that doesn't always mean super-low air temps.
The variable speed blowers are a nice feature, and use a fraction of the electricity of single-speed split capacitor type blower motors.
Humidifiers are mold-inducers (and if never cleaned mold SPREADERS). In most of the lower 48 wintertime humidity under 30% (dry, uncomfortable, staticky) are an indication of too high a ventilation rate, which is most instances means your house leaks air like a sieve. Adding humidity to the air then causes higher rates of condensation and mold in the walls/ceiling/attics along the exfiltration path. The "right" solution is to tighten up the place, not add humidity. It's nearly impossible to retrofit tighten up a pre-1990 house (and many newer homes) to the point where indoor air quality become problematic, unless you smoke, cook without running exhaust ventilation, and never run bathroom exhausts, etc.