I wouldn't think you'd be disappointed!... Having gone from my peanut wrx with 300hp to the Liberty, it is a reasonably big step up in every way except for power/agility! Which can still be improved fairly easily!
I would recommend going for the 07+ facelifted model of the Liberty, although you will pay a fair bit more (mid to high $30k's for an 05/06 Vs high $40k's to $50k's for an 07+) you will get a nicer looking front and rear end plus the brembo brakes and 6spd manual box!
That said, if you are not overly fussed about having the limited edition model, the only difference between the GT Spec B and the Tuned by STi is:
Front Lip
Brembo brakes
STi exhaust
retuned
ECU
Rear lip spoiler
STi embroidered seats
Which can all be had very easily aftermarket(bar the interior), and I would think you would be changing the exhaust again even from the STi
