I figured we could move this discussion.
One of the biggest challenges we have with our Subarus is finding tires that are the right profile and compounds for the track and autocross.
And while there are multi-use tires that will work at the track and at autocross, there are compromises with them.
Lets start with sizes.
For most of us, we are looking for a 17" tire that will fit a 17"x7.5"-8" wheel. So this means we are most likely looking in the 225-245 width range (255 and even 275 may be possible but really want a wider rim.)
So for sizes we have
205/55-16 (Std WRX OE size)
225/45-17 (Std STI OE size, reference) [Good choice and shold fit without clearance issues]
235/40-17 (2.3% smaller) [Better gearing and shoud fit]
245/40-17 (1% smaller) [About as big as will fit most. May require a little fender massage]
245/45-17 (2.8% larger) [Not really a good choice IMHO]
255/40-17 (.2% larger) [Fenders may need more than just a massage to make these fit]
Now we have tires...
Autocross ST legal tires. Size ($price),... [Sources]
Bridgestone Potenza RE-01R: (Proven Championship Autocross tire in STS)
225/45-17($173), 235/40-17($183), 245/40-17 ($19
, 245/45-17 ($200) [TireRack]
Falken Azenis RT-615: (Proven Championship Autocross tire in STX)
225/45-17($10
, 235/40-17($119), 245/45-17 ($117), 255/40-17 ($127) [Vulcan, Discount, Edge]
Yokohama Advan Neova AD07: (Proven Championship Autocross in STU)
225/45-17 ($186), 235/40-17 ($197), 245/40-17 ($21
, 255/40-17 ($225) [Vulcan, TireRack, Discount]
R compound Autocross only
Hoosier A6: (Proven Championship Autocross tire)
225/45-17 ($219), 245/40-17 ($225) [TireRack]
R compound Multi-Use
Kuhmo Ecsta V700: (An OK but not exceptional tire)
245/40-17($195) [TireRack]
Kuhmo Ecsta V710 (Almost as good as the A6, but size is not great.)
245/45-17($177) [TireRack]
Kuhmo VictoRacer (A classic performer)
225/45-17 ($165), 245/45-17 ($157), 255/40-17 ($172) [TireRack]
Nitto NT-01 (New unknown)
225/45-17 ($164), 245/45-17 ($174) [Discount]
R Compound Track
Hoosier R6 (Insanely sticky but short life)
225/45-17($219), 245/40-17($225), 245/45-17 ($225) [TireRack,
Toyo Proxes Ra1 (pretty sticky and lasts...)
235/40-17 ($172), 255/40-17 ($167) [Edge
There are other brands and tires, but bang for your buck and pure outright performance, these are the common competitors... I'll add them if somebody wants them.
So what does this mean... For Autocross, you are pretty much stuck with $200+ Hoosiers to be competitive on race tires, or $1?0 615s for Street Tires.
For the track you have lots more choices, though you really can't beat the RA1s for bang for your buck. The 615s can also be run at the track and hold up pretty well.
With the exception of the Hoosiers that effectively start as "shaved", you will get better performance in the dry if you run the tires at 4/32" or less to start by having them shaved. This will not significantly decrease tire life as they will likely wear more evenly.