Don't know if those will fit but one of the downsides going to a larger (i.e., taller) tire is that your speedometer (and odometer) will be off. Indicated speed on the screen will be
lower than actual, as well as your indicated mileage on the odometer. In other words, you might be speeding when you think you're not and if you permanently change your tire setup, you might get a slightly better price for your car when you sell it. Granted, the MPH error will probably only be 2-3MPH.
Hmmm.... From what I can tell, a 235/45/18 tire has a diameter of 26.33" and a sidewall height of 4.16". A 235/50/18 is 27.25" and 4.63". So yes, you would gain about a half inch in sidewall but also have about a 3.5% error in your speedometer / odometer. That doesn't seem to be too much of an increase but I'd still go to a tire shop that either has previous experience in mounting tires of this size on a 3, or is willing to spend some time and effort into figuring out if it does fit.
I would normally suggest using the +1/-1 tire concept, increasing or decreasing rim diameter and changing the aspect ratio of the tire to keep the same diameter, but it seems like a 17" rim will not fit on the 3.
edit: Poked around the interwebs and lookie here:
Will 235/50R18 snow tires Fit a M3? Bottom line: looks like 235/50/18 will not fit (post #6).
