I have pretty much the identical experiences with wrk101, right down to the model(s).
There are, in fact very few frames built for 27" tires available, actually that were even made in a 22" frame. Back then, Co.s jumped a full 2" in choices. Having contended with this years ago made me sensitive to sizing.
Not these days, not with the availabilty in 55 & 56, bikes with 700 wheels. I like a few bikes that have 55 CC, LeMonds with 55CT on account of the long top tubes.
Stems help but using a setback seat post on a 21" bike works for me. In the past and occassionally these days, I take a ride on a 57/58, it's ok. I prefer the others as I like the feel of having a not so big bike beneath me, depends on how I feel.

The "sweet spot" is most definately NOT your imagination. Bike brands vary in that regard as do the way that they're exactly meas. by the Co.s (beyond just CC or CT).