Would this be an upgrade for the GTI or would it fit? Never researched the topic...
It's a direct swap. I put one in my GTI not to long ago.
How have the results been? I've been interested in this type of swap too.
I don't have IAT readings but I can tell you it's been noticeable on the butt dyno. SC summer heat will hit 93+ and with my old IC I could feel the boost trail off. With the R IC it still trails a bit but no where near as much as the stock did. If you can do it yourself it's a good stop-gap solution until you decide on a FMIC.
Overall, I'd do it again if I got a new GTI.