If the engines are auto-pressurized, I am assuming that they don't produce anywhere near the same pressure as turbo pumps would. Does that mean the pressure in the combustion chamber has to be relatively low?
Yes, the pressure must be low because otherwise the tanks would be too heavy. Especially with such fluffy fuel. (Or then they have some new supermaterials. In theory just the engine pressurization type does not limit pressure. If you could make a rocket tank out of pure graphene, then why not pressurize it very high?)
Saturn V's F-1 or Falcon 9's Merlin are moderate gas generator turbopump designs pressures around 7 MPa. This is probably closer to 3 MPa, I'm just guessing here though. I wonder about their suspiciously high sea level specific impulse, 305 s. They probably have some aerospike extra there.
That said, I don't know what they have really built. I see their California office is a few hundred meters down the street from SpaceX.
Carmack has a more "hands on" approach: http://armadilloaerospace.com/n.x/Armadillo/Home/News?news_i...
While description of VaPak is more technical http://www.holderaerospace.com/downloads/Technical_Papers/Va...