Bard - hope by now you took the advice to buy a second set of wheels and not remount tires. I'm heading into my first winter with the Focus Titanium with handling package - I live in northern suburb of Chicago.
I ordered Michelin X-Ice Xi3 in 215/55R16 XL, added the TPMS sensors and bit the bullet and bought the reset tool (got a call from Tire Rack describing the problem they gave me the option of having dealer reset the TPMS senors or get the reset tool - figured the tool was worth buying since you have to do this reset every time you swap the tires).
I agree with you on the look of those 18 inch wheels, they are super hot, and really make the Focus look different and good. I'm there with you 100%. I also love that 40 ratio tire look. Very darn cool.
What changed my mind was looking at wheels and tire sizes. I happen to have a 1999 Mercedes E320 that requires snow tires. Turns out the wheel and tire size is exactly the same as for the Ford - 215/55R16. Given that my Blizzaks are almost 13 years old, I just bought 8 new Michelin's. When the Mercedes gives up the ghost, I can always use those tires on the Ford.
Sorry - long way around to say this - that tire / wheel size actually looks pretty darn nice. Not as cool as the 18's, but boy it is super close. 15's look terrible - way too much tire compared to the 18's.
The other great thing - there are so many different 16 inch wheel designs. Take a look at the Ford website - pull up that 18 inch wheel design, and compare to what is available. There are some wheels that are darn close at Tire Rack. There are also some really similar wheels to the SE alloy wheels.
We decided to change things up a bit for the winter and went with a black wheel and milled silver face on it. I think this is going to hide dirt a lot better. Still have a lot of open wheel look. My Focus is the dark gray, so that black will probably look good. I'm looking forward to mounting these on the car.
Good luck with your choice on the winter tires.