It really doesn't like to go below 450psi or so, from when I last tested it out. Below that it starts having issues, and I can't recall even getting one to function at all under 400psi. It's been a few years to be honest.
The issue is twofold. Powertube ID and spring tension.
You can only get so much flow through the powertube, and as you lower the pressure you need to increase the time that the cupseal is open. This means moving to a lighter valvespring, and usually a lighter mainspring too.
But if you go too light on the springs and pressure, a couple problems start popping up. First is that you get 'nelson bounce' on the hammer. Remember, the cup seal depends on both spring pressure and air pressure to maintain a seal. Softer springs and lower pressure allow the weight and momentum of the hammer to have a greater influence and it starts bouncing on the powertube. This turns your efficiency to garbage real fast once it occurs. You can somewhat compensate for this by lightening the hammer, but there's not much hammer in there to work with.
Then you start running into recocking issues, as again the cupseal is dependent on air pressure to maintain its seal. Not enough air pressure backing it up means it can't fully resist the force of the mainspring and instead of cocking the hammer, you just force open the cupseal.
If you wanted to go for a low pressure setup, you'd have to turn a powertube with a larger ID, then open up the hammer and bolt to match. Not so easy on a Phantom with the bolt design it runs. Even then, it's a crapshoot that may not pay off.
Nelsons just really like high presssure.