Of course the PSU will see the 208V L-L. But if it's floating, my concern is the phantom voltages that can disrupt interconnections between computers on different UPSes. Just a wee bit of hum on the ethernet connections can ruin their performance.
3pY would work better in the sense that it has to have the ground reference. But UPSes which output 3pY seem to no longer be made with less than 10 kVA. One rack cabinet doesn't need that much power. Not only is there a premium to the cost for 3p, but also for the excess capacity. That, plus a distribution box wired to balance the 2w1p loads across the 3p would be hard to find. 3p is just not commodity enough to be cost effective.
