Subaru is very shy about letting people drive the STi without serious intent to buy because every mile on that car before it is sold is likely to be abuse. It is not likely that anyone new to Subaru's STi is going to shift precisely, brake and downshift appropriately, etc. They are going to TEST the car... see what it is capable of. In other words, abuse it. No one wants a $35,000 car that has already had a beating handed to it.
I got a used 2002 WRX with 80k miles. It was $10k. I put about another $15k into it, and there aren't a whole lot of 2002 STis that can keep up. It came with a glazed clutch, a couple of missing teeth in the tranny... after about $5k into the tranny and diffs, I can launch as hard as I want on 18psi. at a hair under 300hp, and I get about 32mpg highway on 93o pump gas.
That difference has changed. The newer WRX uses a 2.5 liter block with a more developed turbo system. There are more similarities and less differences. This due partly to the fact that Subaru recognizes that the new WRX is pretty tame compared to it's GC8 and GDA/B cousins. The new STi holds those concepts and displays them well.
If you are planning to turn a WRX into an STi, forget it. Hybrid engine builds, stuff like that, sure. If you are inclined to do that. That can be successful, but I am not sure how that would work out on a newer model. I believe the changes to the design have made the cars less "swappable".
One thing that really would help is if you pull into the dealership in an STi. If you already have one, why wouldn't you want another?