This is a good test case for you. You've raced you mustang before, but this will the first on track experience with the STi.
You've got the basics addressed, but I didn't see anything about brake lines or brake fluid. This is a must if you are planning on tracking the car. The lines will heat up and swell on the vicious braking you are going to be subjecting it to. The fluid will boil on the 2nd or 3rd lap. Please be sure to correct these items if you haven't already done so and I just missed it.
I also may have missed if you have a short shifter and hard bushings installed. This will be better too as under pressure at high speeds, the downshift from 5th to 4th can easily result in a 5th to 2nd downshift. Not a good thing of course, but fairly commong due to the shifter angle/tension and the not so great stock bushings.
Tires...stay away from the 18s on the track. The stockers have plenty of grip...just pump them up to about 50 psi and ajust to you driving style as necessary.
All the info above regarding the turbo and GT models are correct. If the driver has half a clue, you will see them from the rear, if at all and that will be it. Unless passing is allowed, then you'll get to see a front a side shot as the sling by you. No comparison at all. Although I do have to edit this.....I've taken on and beaten 911s before...quite a few times...but I can't help but think it was the driver I beat and not the car. I have not had the priveledge of tracking one so all I can do is go from experience.
Since this will be a 3 day weekend event, they will likely impose a wave to pass rule at designated areas only....wouldn't want to crunch up those pretty bodies right
VIR is a techically challenging track. Do you know what paddock you will be using? Just curious as this can make a difference in initial lap. My bet will be the south paddock as it give the long straight first, and provides an opportunity for the Porshes to stretch their legs.
A key item with an STi against a porshe. With good pads, fluid, and lines, you can outbreak a stock 911 (believe me...been there and done that). I'm not sure about your aftermarket pads as I only use Ferodo's. I've tried hawks before and just didn't like the results. The second important item is to remember to take advantage of the STi's difference in cornering. You will be able to apply more throttle earlier on in the cornering line due to the AWD, so use it to your advantage. I've found late apexing works this advantage to it's fullest, as it give you the opportunity to apply the throttle even earlier.
If you were planning on going Cobb stage 2 by race day, by all means get it before then so you can experience the difference first, as it changes the driving dynamics of the car. Stage 2 can easily put you off the track via a 4 wheel power slide if you aren't careful. I found it fairly easy to spin all 4 in second gear coming hard out of a corner, so you need to be aware of this.
Regardless of what you do, I hope you attend just for the fun of it. Next to teh sex, there is nothing better than the freedom of a track day in a performance machine. And you are going to get 2 days of this....with your wife at that I'd say that is a 2fer anyday!