I'm trading in my 2012 SE/Sport on an ST later this morning. I've got a set of Sport Edition 17x 7.5 rims I'm going to put on the ST, and a set of 215/50R/17 General Altimax Arctic snow tires (all from the Tire Rack via a local tire store as well as being leftovers from the SE/Sport).
The 215 fifty series snows have the same tire diameter as the stock ST 235/40R/18s (25.4 in). So my speedo shouldn't be out of whack (they're calibrated for tire diameter). Good luck with your ride!
I'll post again and let you know how it went, and hook up a pic or two, if you like, of the new winter set up after I get it done.
2013 ST2-Tux Blk, FSW 93 Oct tune & 3" cat back, Steeda R sway bar & CAI, Sparco rims, Luminix Ti White bulbs
2007 ZX14 Sato Ti exhaust, PC III, CF bits 'n pieces
2011 M109R buncha stuff