Larger tires stock 18's

[TPrelude-370z]

Hey guys,

I just got my 2015 about a week ago, and I need some quality all season tires for the rear. I know, all season = no season, but I'm in Chicago and I only have a couple months of a light winter left. I only live 1 mile from work, so I just need the rear tires to get me moving. They would also be helpful for better grip on the cold roads as well. I won't ride my Z as much in the summer because I have a motorcycle.

Here is the plan: Stock 18's and stock tires

Move stock rears to the front and buy x2 g-Force COMP-2 A/S 275/40/18's for the rear. The only other options for that tire is 255/45/18. They seem to get great reviews even in light snow.

My questions:

Will I get better grip with the wider rear tires? Especially on cold streets.
Will the driving/handling be drastically affected?
Will this look good on stock suspension (fitment/fender gap)?
When I get swift spec R springs, will these tires still fit all the way around?
Can I still add spacers with that set up (including springs) without rubbing?

Any thoughts? I plan to purchase the tires pretty soon. Once summer hits, I'll probably sell the Advans, and grab matching fronts.

Also, if you have better options as well as spacer specs, feel free to chime in.

Thanks guys.