I'd invest in more seat time alongside or before any serious mods. If you like the mods though, I don't think they will hurt but you will definitely go faster with more seat time.
There's another event at Thompson with the club I instruct with in a couple weeks, they run a great event and have good instruction if you want to check it out.
http://www.comscc.org is the link.
I have done 2 events in my 135, which is bone stock. I ran NHMS chicane-chicane in July, and Watkins Glen this past week. I really don't see a lot of what folks are complaining about with the understeer and tires. It's certainly not perfect, but I have had stock cars that were much worse off (my '05 and '11 STIs were understeering pigs in stock form). My E92 M3 was more balanced out of the box, and had much better brakes and grip, but it also had better tires
That said, for runflats the stock tires weren't awful, but I think better (and wider up front/all around) is a great idea.
Out of the box, I thought the car handled a lot better than I expected. In terms of tire pressures, I found more grip running at about 38psi hot than I did at 40psi hot. The car seemed to pick up about 7psi during a session, so I would go out at about 31psi cold.
I did not notice an excessive amount of understeer; quite the opposite, I thought the car was pretty well balanced out of the box. I did have it aligned in July prior to going to NHMS, so I am sure that helped. I had about 1/16th toe out in the front, zero toe in the back, -1.4 camber in the front and -2 camber in the rear (I drilled the front alignment pins prior to the alignment). Where I did see some understeer was in 2a at NHMS (more my driving than the car, I tended to not slow enough) and on occasion in T1 at the Glen (my line I think had more to do with it more than the car). Those are slow to mid-speed turns. High speed understeer I didn't notice a lot of.
I do agree with the car having too much power and not enough tire; the skinny front tires hurt overall grip and braking, and the rear tires just can't cope with the torque. At NHMS t3, the car was really fun, but slow, going up the hill sideways.
This is where I have a big complaint on the DCT; even with DSC off, Sport mode on and shifter in the manual position, the car would still downshift on me when I went full throttle. Given the abundance of torque and lack of tire, 2nd gear was bad for lap times so I would use 3rd in t2b and t3 @ NHMS. Half of the time, the car would downshift to second even though I manually selected (and was driving in) 3rd. While I love the full throttle upshifts, this was a real pain in the neck and hindered my lap times. Shifting into 5th, flat to the floor, at the apex of t3 in the Esses at the Glen was pretty awesome... but I'd still prefer the manual.
The one real area of weakness I saw were in the brakes. The front brakes heat up quickly, so pad selection is important. I ran some Hawk HP+, which were not very good. I normally run DTC60s on my race car, but I thought they would have too much initial bite for the street tires, and I wound up with the less torquey HP+. They bite better than stock, but don't deal with the heat so well. I think the HP+ would be ok for the rear but a DTC30 or 60 up front would be much better (or some other equivalent brand). I didn't have a lot of faith in the brakes so that held me back a bit.
The DSC definitely needs to come off if you want to have any chance at a fast lap (in the dry at least). I wonder if having it on helps add to the understeer you saw? If you have trouble keeping the car straight with the DSC off, definitely work on you before the car. You won't be able to take much advantage of better suspension, bushings or brakes if you aren't comfortable with a little slip angle. I don't mean that in a bad way, but I think the oversteer-ability of the car balances out the plowing, but you can't get oversteer with the traction control and stability control on.
I'm not going to the Thompson event, but we have some great RWD instructors going so if you want to hit the event, PM me and I can give you some names to request.