Tire plans are coming together.
I went ahead and ordered some of those discontinued Conti/Hoosier wet race tires, in 265/645R18 (
Hoosier Racing Tire -- Tires Designed For Champions -> Road Racing -> Discontinued -> bottom of page - $100/tire!).
Ordering another set of 18x10+34 Forgestars to stick them on, too. It'll probably be ~5 weeks to get the wheels in, at which point I'll finally be able to swap between decent dry+wet options in the paddock. 18x10 may or may not be the most-optimal rim size for those wets, but it'll be close enough to work, and in the long term I like the idea of having all my rims the same size and then just keeping an eye out for matching tire deals.
On the dry side, I'm figuring when this current set of RS3's run out of steam (probably mid-Summer?), I'll be ready to try some dry slicks too. My plan there is to pick up random brands of cheap scrubs, it shouldn't be hard to find them in 18x10.