I'd like to chime in about "real world" usable speed here.
I've almost purchased an STI on two different occasions (not an EVO tho) and two of my close friends own them. We often go on fairly extended "high-speed pleasure" cruises in Northen California, where the terrain, road conditions, temperatures are extremely varried. I drive an '03 Z06.
Technically speaking, my car is lighter (3100lbs curb), more powerful (405 HP "if stock") and has more mechanical cornering grip than the STI. By all accounts a "faster" car.
In the real world, the conditions really need to be good for me to get the upper hand on the STI though. The gearing differences between the cars are such that the STI will pull from 60 to 100 nearly as fast as the Z, but the Subaru does it in 4th, where I need to hold in 3rd almost the entire way. Off the line, I can get away from the STI, if the road temps are perfect. If not perfect, I'm spinning 2nd and through the bottom half of 3rd and if the roads are even slightly wet, game over.
Similarly, I can carve a much faster line through the turns, as long as the road surfaces are flat and even, road temps good, etc. once things get uneven, the Subaru is a much more forgiving and flexible car. I'm not saying faster, but certainly easier to drive & possibly safer.
I'm sure that in a well contained environment, (drag strip or road course) the Z06 by the numbers would win out, especially stock for stock, but in the real world I think that Subaru and Evo owners have fun, a lot more of the time (for me that is pretty important).
So, why did I buy the Z06? Seat of the pants feel! There is no comparison between the torque delivery of the Z06 and the STI. Also, the Z is very low slung and cornering behavior is very flat in comparison to the STI. The Subaru feels very tall to me and gives the sense of "leaning". The upshot to the subaru is that it does have a narrower footprint and combined with its ride height gives the driver more visibility and potentially better use of narrow roads. Lastly, I like to drive long distances, and the Z is super comfortable (very long legs), and will get me 26+MPG, cruising in 6th at 85MPH!
My .02, bang for the buck, there is no better all around fun car than the WRX/STI (no offense intended EVO guys, I just don't know your car).
Sorry for the rant.
-Gabriel