Well I know they're big in the mustang world (uncle actually uses one on his 08 GT making 515ish to the wheels on the stock pump) and guys have used them with good results on WRX's as well. I wouldn't always use it though, just something about adding voltage to a fuel pump not made for the higher volts.
If you have an aftermarket fuel pump though, a lot of times they do enjoy a higher voltage like 16v.
In the end the injectors are still the bottleneck on the 04's. Those 440's can't keep up for very long, even if you crank the FP up you still only get 500's or so.