I think you're asking for headaches by paralleling power supplies that weren't designed for load sharing. There are enclosed power supplies from the usual suspects that are designed for this (additional signal connections that are used to balance between parallel supplies), or maybe using fewer larger supplies in series and a 48V inverter would be easier. That may even end up being cheaper if you're looking to buy the PSUs new.
A boost transformer solves the voltage problem, but not the problem of total available power at the receptacle. If you're already seeing a nearly 10% drop in voltage at full load, then the additional power required for the transformer is going to result in diminishing returns if it's on the same circuit. A lot of buck-boost transformers have isolated windings so that one transformer can support a number of different applications depending on how it's wired, so
hypothetically with some interlock logic to verify that the phasing is correct you could run the transformer from a different circuit on the same phase. I'm not sure if that would be code compliant, but there's not a
technical reason it wouldn't work.
Correct me if I'm wrong, but isn't 208 VAC 3-phase phase-to-phase voltage in US systems? Meaning they'll be floating and out of phase?
Or have I misunderstood US power distribution?
Not sure what you mean by 'floating', in US 3-phase systems 120V is the phase-to-neutral voltage and 208V is the phase-to-phase voltage, so the 208V circuits are already using two phases 120° apart. If you were to pull from multiple circuits, you would need to ensure that you were matching phases correctly, as well as provide some sort of interlock to prevent output without all inputs connected correctly.