I did a bit of digging, and found what is supposedly a HX35 map, a stock one would be OK for a 15 psi setup, but trying to push 20 out of it would have it outside of it's map alltogether. The 60 mm compressor swap from the looks of it will move 69 lb/minute, and 20 psi on a 3750 rpm 6.9 is 68.87, so it should just be able to pull off 20 psi all the way to the fuel cutoff without going outside of it's map.
I'm having a had time finding good hard info on the Holsets though, as Holset doesn't release their turbo maps, and I'm working off of what I'm finding online.
From the looks of the turbo map I found for the stock HX35, the compressor is sized nicely for a paralell twin setup, 20 psi would put it right in it's peak efficiency range.and you would have room to go up from there.
Going back to the old twin turbo thread and looking at my old calcs, the T66 was the compressor I had picked as best for a 20 psi single turbo app, and it actually seems to get thrown around idea wise as an upgrade on the 7.3 PSD pretty often online.
Have to remember, with either a litre, or 1.4 more than a Cummins, we need a whole lot more turbo to make comparable boost levels.