Just back of the envelope:
- the US 105mm gun with garden variety AP was credited with penetrating 177 mm (7.0 in) of rolled homogeneous armor (RHA) at a 30 degree angle, and 84 mm (3.3 in) of RHA at a 60 degree angle.
- The Maus turret armor was to be 200-225mm, hull was 200 frontally, 180 sides, 150 rear
So if the US was worried about T28 vs. Maus duels, and fielded a 105mm HVAP/APCR round, then it seems like a T28 could destroy a Maus from any angle out to 1000 yards or so, maybe further. With just AP, at least the hull sides and rear would have been vulnerable, and a 105mm AP that powerful would make a mess of the running gear. I haven't seen a source that says the US actually made HVAP for this gun, but it would have been well within the US capabilities at the time.
Looking at it the other way around, the T28 is credited with armor up to 305mm thick, though it's not clear to me how much of the front this covered and the sides and rear were much less. The 12.8cm Pak44 was credited with better penetration than the US 105mm, but not by a lot, maybe 200mm at 1000m.
The T28 probably has a mobility edge but has a fixed gun, compared to the Maus with less mobility but he ability to traverse 360 degrees (though who knows how long this took).
So, if you postulate a T28 with HVAP, on the defense (where it's low silhouette pays off and it's limited traverse is less of an issue), it's probably going to do quite well. OTOH, if you're thinking a T28 with just AP attacking and a Maus defending, then the Maus is in good shape unless it's unlucky enough to only have a shot on the heaviest frontal armor of the T28.
