I think you need to look at what the average outside temps are doing over a 24 hour period. E.g. with overnight lows around 20F and daytime highs in the 30's but above freezing, the Summit seems be able to keep average interior temps here above 70. But add wind and daytime highs less than freezing and temps will start dropping into the 60's.(but that's well above 60F.) I haven't experienced a January with it yet. But guesstimating from the weather I've seen, it would probably take a sustained freeze of several days with wind to bring the interior down to 60F (without any supplemental heat)
Compared to the Century, it's night and day. The twice or more longer burn times (firebox 2x size) plus higher heat output. That small Century even in 50's weather couldn't seem to bring the average inside temps above about 68, and that was with about 1000 sf in rooms closed off. (but then again until right before I replaced it, I hadn't learned how to get the most out of it: I wasn't filling the box full, the wood was marginal the first year, et.al. and I was avoiding using any backup heat, which I think you have to resign yourself to do, to maintain sanity when the stove can't keep up.)
But if you mean daytime highs in the 20's and overnight lows into the low teens or single digits, with wind, without any supplemental heat, and day after day, then I could see temps inside maybe dropping to 60F.
What it really boils down to is what your house's actual heat loss is, under various conditions.
I would guess the Summit (the way I'm doing it with the mixed hardwoods, pretty dry (but not maximum dry) wood I have, can put out a net average of about 50K BTU/hr over as short as possible burn cycle, (which is somewhat less than the BK king can do, iirc). And my house probably at worst needs somewhere around 65K BTU sustained over a few day period of our most severe weather. Meaning it might actually need as much as 75K BTU when it's around 0F with wind, (but we've never had that kind of severe weather sustained over a 24 hour period, in this climate, in my lifetime anyway.)
So the Summit is probably one of the highest outputting inserts which only requires a 6" liner. I'd guess the Buck 91, Kuma Sequoia, the big Lennox, and maybe a few others in the 3.5 or greater cf firbox size which require 8" liners can output more heat. But if your heat loss is greater than what the stove can do, your interior temps just won't be maintained.
But I would think, you should be able get more heat out of your Appalachian than the 1.8 Drolet, somehow. I believe hearth member, mellow, has the same Appalachian insert. Might want to talk to him about it.