I have many posts on this subject. See links below for several discussions.
http://forums.nasioc.com/forums/showthread.php?t=689502
http://forums.nasioc.com/forums/showthread.php?t=686520
http://forums.nasioc.com/forums/showthread.php?t=648828
http://forums.nasioc.com/forums/showthread.php?t=650224
http://forums.nasioc.com/forums/showthread.php?t=641162
My opinion, and I seem to be in the minority, is that the factory STi wing is not particularly effective. It *looks* like the WRC wing, but that wing has several significant differences from the factory STi wing.
In the grand scheme of things, you are looking at 60 lbs of downforce at 100mph. So 60 lbs on a car that weighs 3200 lbs. In my opinion, you won't even notice that.
I've removed my STi wing and replaced it with the WRX spoiler and am far happier. It weighs much less, and improves rearward vision 100%. I've noticed absolutely zero difference in the feel of the car even at above normal highway speeds. I don't regret the swap one bit.
-Michael