http://www.turbochargers.com/Turbos/...grades/16G.htm
"...at 15 PSI boost Supports 340 HP."
Stage two would probably refer to porting of the turbine housing. Stage three use to mean clipping the turbine wheel.
These are old terms probably first used by Buschur. I suppose most people just say it is ported or clipped now instead of referring to stage 2, 3 or whatever.
And there is no difference. It should still be around 8psi. Problem is if they port the wastegate then it is possible it might be more depending on tune and exhaust parts.