That ad states that Doluck make one which is about USD$800 have you seen these?
Edit- found the Doluck ones they only have them for the GC8 which has a 60mm throtttle body.
http://www.do-luck.com/throttlebody.html
Translation of text-
It expands normal throttle diameter 60 φ to 65 φ, just replaces at entire area both power & torque the aluminum which it improves to start shaving, the big throttle. The throttle linkage transplanting normal that way, in order to use, either accelerator feeling did not change, low cost, actualized high performance.
The throttle body for EJ20K
* IMPREZA Vr.3 - 4 65 φ XXYEN 60,900 (substance price XXYEN 58,000)
* IMPREZA Vr.5 - 6 65 φ XXYEN 68,250 (substance price XXYEN 65,000)