[OkiDori17]

How long have you had the car?
If you've just gotten it, I wouldn't try cranking up the boost quite yet.
If you can't trust the guy knowing one thing, how do you know he installed everything correctly. I would drive it around and make sure everything is running right before I'd go for more power.
But 12lbs is around all I would go. The stock head gasket was made to run 6-7lbs (stock), how do you think it'll like doubling that?
If you have the cash try picking up a good head gasket and not only will your engine be safe, but you might learn a thing or two from replacing it.
A BIG thing I've learned from the SR is it's far better to be safe than sorry, being sorry costs too much. =]