Zanx,
I don't/won't try to explain the scope, but I have been reading up on the Stealth's.
The puff of air around the breech is not a desireable thing. From what I understand it is a leak from around the valve stem that vents to atmosphere and thus is robbing you of power or wasting air. On one of the Stealth sites, this was overcome by machining an O-ring groove inside the valve body so that the extra O-ring ensures all the air from the bottle goes up the barrel.
Whilst I cannot say for definate that this is the case, I would recommend a trip to an RFD to get it checked/repaired and set to sub 12 ftlb.
HTH
Ogri the trog
Improvise, Adapt & Overcome