I wanted to go from a T28 to a T25 on my new daily, so I bought a junk T25 to get rebuilt. When I sent it to G-pop Shop to be rebuilt, they said the turbine wheel was damaged and needed to be replaced. A new wheel would have bumped the rebuild cost up to about $550, which is a lot to sink into a T25 rebuild. And if I did nothing I was out the cost of the blown turbo, plus an "inspection fee" and shipping charges. In the end I decided to just spend a little extra for their T28 upgrade, which was $720 with return shipping. That includes machine work, replacing the entire center section, and replacing both wheels. It's not from a GTIR, but it's a Garrett center section. The plate # comes up as a GT25 on a John Deere, lol.
Anyhow, at stock boost I really can't tell any difference from the T28 to the T25/T28. I'm sure at higher boost levels a T28 would outperform the "Big T28" but it's fine for me.
There's a bunch of pictures in this thread:
http://www.sr20-forum.com/turbo/9336-gpopshop-com-big-t28-upgrade.html