The stock offset on the front 8.5" wheel is +25... assuming you don't want to go narrower than 245 (stock front), keeping the wheel width and offset as close to stock as possible is ideal. To keep overall wheel height the same, for 19" wheels you'll need 245/45R19 tires and for 18" wheels you'll need 245/50R18 tires.
Someone here already fitted 18x8.5ET35 wheels on an SS, so looks like the extra +10mm offset doesn't disturb the 4 piston brakes... but I think the wheels sit too far in that way; however, finding 18-19" wheels with +20 to +25 offsets is nearly impossible without going the more expensive custom route.
I have the 6 piston brake package, so 18" were out of the question. There's just under an inch between the widest point of the caliper and the edge of the wheel. I ordered a set of 19x8.5ET35 wheels, Eibach 15mm spacers, and Dunlop 245/45R19 winter tires... they should arrive by the end of the week and I'll post all the pics.
Overall wheel sizes:
- stock front - 704mm
- stock rear - 700mm
- 245/45R19 - 703mm
- 235/45R19 - 695mm (8-8.5" wheel possible)
- 265/40R19 - 695mm (9-9.5" wheel possible)
- 245/50R18 - 703mm
- 235/50R18 - 693mm (8" wheel possible)
- 265/45R18 - 695mm (9" required)
The tire size I chose (245/45R19) had a decent selection on TireRack... so check what's available in terms of winter tires if you're interested. I wouldn't want to buy some oddball set that'll be discontinued in a year or two.