Anyone considering this upgrade, most kits include 2 calipers, 2 silencing clips, & 4 slide pins. You'll need rotors which don't change and pads for the 06/07 WRX. I'll edit this post and attach a complete parts list when I get home so you can check off.
In addition, you'll need these items. I don't know why they don't include this. It prevents the pins from backing out, even if you were to get aftermarket pads. Cotter pin is very important. The shims aren't so much.
cotter pin
26232 = 26232FA010 (2 needed) List Price: $4.58
shims
26233D*E = 26233AE010 (4 needed) List Price: $4.08
26233D*C = 26233AE050 (2 needed) List Price: $4.08
26233D*D = 26233AE020 (2 needed) List Price: $4.08
Photo reference:
Helpful referenced source:
'06 4 Pot Front Brake Pad Tutorial - NASIOC