The sizes you're talking about aren't a problem. I've run 275/60-18 square for quite a while, with zero issues. Some others have, too, in the past. Your 285s are basically the same thing.

The front to rear differential is your risk issue. I haven't seen anyone talk about running a diameter differential like you are. I'd suggest you pick a size and go the same at all four corners.

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2013 X5M - Sapphire Black
2006 X5 4.4i - Sapphire Black
2002 M3 Euro - Alpine White
2003 M5 - TiAg
2003 M3 - TiAg
2010 535i - Deep Blue Metallic
2007 530i - Deep Green Metallic