MB is the same bolt pattern ,but check the offset !You will not want a much smaller rear size maybe 1/2- 1" overall max.That won't make that much overall difference.Remember 1" overall = 1/2 " ht difference.Tire width won't make alot of difference.I'm sure you can find a nice combo.
I think if you want a nicer ride you'll need 16 or 17" wheels.

 

the bolt pattern is same as MB, 5x112. The offsets ARE NOT the same. Xfire rear offset is 22mm on 9" wide rim. MB does not use these offsets in the rear... Diff offsets along with wheel width changes will change the track of your vehicle. Which may change the handling of your vehicle...just be aware of these issues.

 

Aw, dash it. I've been driving my xfire very spiritedly lately, it's just broken in, and I can't see changing the wheel size anymore especially if it means less grip and road feel. Silly idea in the first place...

 

The wheel wells are made for larger wheels like the stock ones and Im thinking any smaller would look funny. Of course you could use high profile tires but then that would really be heavy.

I love the stock wheels and consider the car a work of art. I would however love to replace the 'plastic' grill and a few other things of that nature but the wheels are perfect for this car.