11" wide is getting pretty close to the widest you can fit back there without having to flare the quarter panels or necessarily having to modify the shipping tie-down brackets for ease of tire removal, but it is possible. I have 18x11 Forgestars and I did do the tie-down bracket mod (I left the portions that the bump stops use).
At the rear, it's very close to flush with 285/35-18 tires.
There is no way to fit 11" wide wheels or tire sidewalls fully under the sheetmetal up front with conventional struts (Cortex carries some one-off struts that might get 11's under the fenders). But if the tread isn't much wider than 10" at least the tread will be under the metal and you won't get too much trouble it with pebbles tossed up by the tires. 19" vs 18" won't make any real difference and there isn't a lot of wiggle room to juggle offsets either way.
Here's the closest picture I have to looking straight along the side of the car. Looking straight down, if it sticks out at all, you can't tell just by eyeballing it, it really is that close. BTW, axle centering gets very important. If I wasn't actively road-course tracking the car, I probably wouldn't have gone quite that extreme.
Norm