Have you checked out this thread?
Topgunz - air to air upgrade
Stillen is OK for what it does, but there have been many discussions here about its shortcomings -- the main one being the flow restriction and poor heat exchange of the supplied intercooler.
If I understand correctly, even if you don't increase boost, you'll notice substantial improvement by going A2A.
In addition, Stillen's tune is apparently terrible. A better tune would most likely give you higher numbers.