the factory turbo is pretty much maxed out in terms of efficiency where they set it at from day one. you will and can make more power above stock boost but its progressively less efficient the higher you go. you honestly stand to see better gains by doing some upgrades to the car at stock boost than you do by simply cranking the boost up.
When zigger did his tuning he told me 17 also on the stock TMIC and 18 on an upgraded IC. Considering he just did 12.6 at 108 in the qtr I think it's working.