It does work better, no question. Since the factory duct on the WRX looks fairly small in cross-sectional area and it just dumps air into the fenderwell area my guess is its effectiveness is marginal at best. To really get a noticeable effect you need to run ducting right up near the center of the rotor to blow right into it... this works pretty well and is also totally do-able in your garage. For maximum effectiveness you need a close-fitting backing plate that isolates the rotor center from outside air & insures all the ducted air from the front will be directed into the rotor... this isn't very easy or cheap to do though.
There's a guy on CC.com custom TIG welding up these for about $250 a pair...
