I installed the rear part of the kit on my WRX last night. I started with the rear, and actually that's all I got done so far...The removal of the stock parts took all of about 5 minutes, however snaking the new bar in, that's a different story. I am not a puzzle guy, and I should have taken more of a note as far as how I was able to remove the stock bar. I spent about 30 minutes trying to snake it into place, different ways to put it in, to no avail. I do have the SPT exhaust, which it would bump up against, so i'm not sure if that was a problem or not, but it was PITA. I ended up jacking up one side of the car to decompress one of the a-arms, and after gaining about 1" from that, it slid into place. I then re-compressed that side, let it settle in, and attached the endlinks, and d-brackets. I torqued it to spec as to not snap any bolts off. I actually didn't have too much trouble coaxing the end links into place, and I put it on the stiff setting. After completing the rear, and it being about 10 at night, I decided to wait on the front and took it for a spin. WOW what a difference. Way less body roll, and much less under-steer as well.
I plan on installing the front this evening. Hopefully will go a bit smoother as I don't think there is much snaking/puzzle type stuff to get the bar into place. I am interested to see if I will notice as much of a difference with the front as I did in the rear.
Another note is on my commute into work today, I didn't notice any difference in the ride quality. Not any stiffer or anything noticeable.
I ordered the 7718.320 kit off of tirerack, and got it in about 2 or 3 days (very quick). When I opened up the kit, I noticed the rear bar's part number was 7717.312 on the install paper, and it also has 08-09 WRX listed. I called Eibach and they confirmed this is in fact the correct part, they just hadn't updated the install paperwork. The front bar is also 7718.310, but they confirmed that is also the right size for my model year ('12, they don't have any info for the '13 yet, but its the same).