If you really want to read the STI threads:
Thread 1,
Thread 2.
After reading first thread entirely, and second thread half way here's the short version: Car feels more alive, clutch engagement (shifting) is smoother,
dyno 1,
dyno 2 show about 5-10hp/tq increase (more on the second vehicle which is
modded). Some people had issues with vibration, turned out to be a difference in driveshaft components for 04-05 cars; all the issues were worked through with actp and I think everyone ended up happy, one guy that wasn't happy turns out his diff was out of alignment.
Here are results on a Z28.