I put my Pioneer AVH-3200BT in my STI last night. First off, it drives the stock speakers very well. The stock stereo would distort quickly (though it sounded ok at lower volumes). The Pioneer doesn't distort - just sounds amazing. The stocker also sounds terrible when listening to the radio - it only sounds decent with listening to an aux/cd input. The pioneer sounds great either way.
Some informational stuff for those who are going to do this:
1.) I couldn't find an easy way to wire the e-brake, so I went the "bypass" route. Worked very well. For those interested - this is a great "how to" specific for the Pioneers:
You can not just ground the connection - it won't work.
2.) If you buy the 20pin connector from Crutchfield, or a local shop - they don't seem to pin out the antenna power conncection. If you look at the link below, it is pin 9:
To get around this - I just wired it to my remote turn on and it worked fine. I didn't do this in my last car - and couldn't get a radio station to come in even if I was sitting under the stations broadcast antenna. Now my reception is as good as the stock stereo.
3.) I also installed my BOSS Audio low profile 8 inch sub. I placed it under the passenger seat just as I did on my 07 WRX. On my 2007, this added a lot and sounded great. I think the issue for me now is the seats sit lower to the floor than the 07. The BOSS needs a certain amount of air clearance between it and anything above it if you really want to drive it hard - otherwise it resonates and sounds like a blown speaker. I have left it in for now, but I have had to turn the volume so far down you barely notice it is on.
4.) Lastly... I don't know how long I will keep this stereo in. Even though it sounds so incredible - I now have an iPod wire hanging out the front of the stereo since you loose your armrest connection. Also, the angle the stereo sits has really bad glare. When the sun hits, you can't see any of the screen. I am going to look around for an anti-glare screen to see if that helps at all. I will update if I find one that works.