I was looking at the Agency Crank Pulley online and I was wondering if you only really get this if you have a supercharger or if you can install it N/A.
And if so, what kind of power gains are there?

All you need to know:
http://www.scionlife.com/forums/viewtopic.php?t=248746

NST makes some great products. Even if you still want the Agency one though, this thread will answer a lot of questions for you.