To test for clearance put your rears on the front and see if the 285s fit.
255/60s might be a bit overzealous for making the tire a full 2" taller, I would start to be concerned with contact with the fender well.
Wife's 50i handles perfectly "ok" with square setup 255/55/18 so with your mindset of ok to trade comfort for handling I wouldn't worry at all about upsetting the delicate balance of handling.
One more square setup to consider.
265/55 will get you 14% more sidewall, 5.2% bigger.
With that size your speedo will be off 1mph low at 75 (actual speed 76) but will be spot on at 57.
(The green line is original tires actual speed, black line is speedometer speed)
That option is your max comfort option and the X5 has very large wheel wells I don't think you will have any concerns about making contact.