Blower door tests give you the order of magnitude, aren't a precise way of determining the infiltration losses since it actually matters where those leaks are. (I hope you chased around during the test and figured out where the worst leaks are so you can remediate?) 2816 cfm/50 isn't super tight. It's also not super leaky about 2 air exchanges per hour, but it's usually cost effective to improve upon that a bit if you found the leakage paths. At 1500cfm/50 and lower the leakage is often harder to find and fix.
Using really crude "N-factor" modeling, in your area you can divide the cfm/50 by 17-20 for the max infiltration rates to expect. So if we divide 2816 by 17 (the worst case) you're looking at 166cfm infiltration in a high wind, which is 9960 cubic feet per hour. At a delta-T of 60F (10F outside, 70F inside) the dumb heat loss number that adds up to is 9960c.f. x 60F x 0.018BTU/c.f. per degreeF= 10,756 BTU/hr.
In reality it's literally never that high, since there's a "heat exchanger effect" along the infiltration & exfiltration paths, but it might be as much as half that figure- call it 5K. To be the full 10,756 BTU/hr requires that the leakage paths be large and low-impedance, so that the volumes of air entering the innermost skin is truly at the outdoor temperature, and the air leaving the outermost skin of the building is at the interior air temperature. It's still reasonable to go with a Solo 60, and continue air sealing.
The 115KBTU/hr IBC is really extreme overkill here, but if the installer seems competent on the system design, it'll probably give reasonalble service, and the boiler & system should be tweakable to modulate most of the winter.
