Generally I don't recommend bumping up a tire profile, let alone two. It'll dramatically throw off your speedometer and the overall circumference and diameter of your wheel. Cross reference
http://www.1010tires.com/TireSizeCalculator.asp and punch in the tire sizes. You'll see the overall diameter change and speedometer difference. You'll want it within 3% of your OEM sizes. If anything, I just recommend moving from run flats to non runflats. Like everyone stated, it's a night and day difference and I'm sure you'll be happy with the smoother ride. 245/45/18's would be huge on the 1-series.