FWIW, I went with 205/45-16 and235/40-17 R1Rs for this year. I've still not figured out the magic handshake for this car, only been to one event this year, went sloooowwww.
Just got my Ohlins back on, now with legal perches (yay!), put caster in the car, finally. My car had all the shims on the backside of the control arm, moved them to the front. Last event I ran the car at, it had good steady state behavior (smaller BWR bar on full stiff), but HORRENDOUS turn-in - I had to slow wwaaaaaaaaaaaaayyyyy down to get the front to bite. LSS shocks.
Kinda felt like too much bar, but I found a midcorner picture that suggests it does not have too much _roll_ stiffness, just zero bite at turn in. Rear toe is ~1/8" out.
Match Tour this weekend. We Shall See. Ohlins, left the bar full stiff, grippier site, I _hope_ the car turns in. It should.
I also have to get used to downshifting to 1st for slow stuff. Hrmph. Should be less of an issue at this weekend's site - much, much larger, but the car is so dramatically faster "on the cam" than off, 1st gear/keeping it above 6200 is important.
Should be fun. From what I've read/people I've talked to (pretty good sources...), the "magic" tire is simply good rubber down to the cord - nothing different about the _tire_, just the compound. The 195/50-15 and 235/40-17 are two of the "magic" sizes; there is a third, but I don't know which.
Way I figure, the rears should last for two sets of fronts. I hope. This weekend's site will be a good test - it is harder on tires than the other event I've run them on.
Once I got the car turning, though, lateral grip _felt_ great. No data to back it up, but lateral did not feel substandard - just no initial bite. I'm HOPING the caster fixes that.