If you want tremendous dry tires at reasonable prices, look no further than Kumho Victoracer 700. I have used them on my Cobra on the treet and the track. The 275/40/17 I get cost about $145, I imagine the STi size would sell for less. They are DOT approved, but I would recommend very carefull and slow driving on the wet (they are almost slicks). Not exactly an STi kind of tire if you ask me, but they are amazing on dry road.
the Kumho V700 is not
a viable street tire if you are going to do any large amount of mileage. they're going to wear out FAST. the RE-070's may be a near-track tyre, but they're going to last lots longer than V700's run ont the street. I also don't think it's even remotely safe to be driving an SVT Mustang (or STi for that matter) on the street at speeds where you're going to be using even 75% of the stick of R-compound tires. hit a patch of gravel & you're off in the ditch/on-coming traffic/trees/etc. driving that hard is for the track not the street.
I suspect that if you drive hard on the street the RE-070's will likely last 10-15k. driven easier, they might last to 20k, but very unlikely. saving the OE RE-070's for the track seems like a good idea, as long as you realize the car will be quicker reacting on the RE-070's than any other tyre you run.
initially I'll likely put on some old 225/45-17's I have from my previous car for messing around on the track - less stick = lower limits = better learning environment = less of pricey RE-070's scrubbed off on the track. also, I have a couple of friends who have extensive track time in WRX'es who will likely play with the car on track & I'd rather they burn up the cheap - hell, they're free since I've had them so long & have no use for them - tires instead of my fancy OE ones. I'll likely not track the car that much either, so maybe I'll use the RE-070's on the track & the street. we'll see.