Welcome to SubyClub mate.
I currently own a 2006 Liberty MY06 3.0R SpecB wagon (click the link in my signature to see some pics) and I must say it's the best car I've owned. It is powerful, has plenty of guts and does everything I want it to do with great ease and efficiency, and I've got the Sportshift 5 speed automatic. I have driven to the Hunter Valley with it, towed my cousins boat without a hassle at all and use it as my daily drive. The fact that you're going for a wagon means that you'll have plenty of space for outdoor activities, shopping, parts, towing boats/trailers, etc. Pretty much anything you want to do in day to day life will not be too much for it. You will definitely be happy with the upgrade, and the Gen IV is by far the best looking Subaru in my honest opinion. I rate it better than the BRZ.
In saying that though, I am unsure about the 2.5 GT SpecB as I have not driven one. The turbo would be awesome, but there is nothing wrong with turbocharging or even supercharging your 3.0R.
Tuning it would be no problem at all either. As a matter of fact, once I get a new exhaust installed I will be going for a tune myself.
You're gonna get mixed emotions from different people on whether to go for the GT or 3.0R. But judging from the majority of people that have expressed their opinions, the 3.0R is the way to go. In regards to the difference between the Liberty and the Outback, well I went for the Liberty as I fell in love with the look. I'm not a fan of a tall car and wanted a sleeky and sporty look even if it was to be a wagon. The Bilstein suspension system that comes in the SpecB's are bloody amazing and the ride is comfortable and smooth. I've noticed no problems whatsoever other than simple maintenance and/or replacement parts due to age, but you're going to get that with any car.
Possibly the only con that I've noticed in my wagon is the fuel efficiency. At the moment I am running at 12.2L per 100 km (I always fill up with 98, nothing less), so yes, it is a little thirsty/expensive (especially these days) but I've been told once I get it tuned, that number will definitely lower to anywhere between 8-10L per 100 km depending on the tune itself.
You've chosen a perfect car to go for and I'm sure whichever one you go for, you will definitely be one happy chappy. Best of luck mate and by all means ask more questions. The more research and knowledge you get, the better. The blokes on this forum are a HUGE help and assisted me in all aspects when I bought my car last year.