STI OE TMIC would be a mild upgrade, but nothing too significant.
A good aftermarket TMIC, on the other hand, will cool the air better, which allows you to tune the car more aggressively, while maintaining a comparable safety margin. It will also have a lower pressure drop, which will help spool the turbo faster, offer better response, and not make the turbo work as hard to achieve the targeted pressure.
mainly if you up-grade your turbo you need a bigger inter-cooler to support it. most people run front mounts when they upgrade though. Also EJ257 is right, other mods are more important before you take that route though.