I believe the biggest potential problem with purchasing a 19" winter wheel set up is not so much the offset per se - the problem is the inner part of the barrel, which needs to clear the brake calipers at the front.
Remember, the front brakes are HUGE. This is why BMW has a dedicated set up for the XMs.
You cannot simply purchase a set of 19" rims with the correct offset - chances are, they will not clear the brakes.
I have not seen ANY other 19" wheels that fit the bill, apart from the OEM 298M set up.
I also toyed with the possibility of ordering 20" wheels, and putting winter rubber on them, but in the end, purchased the OEM set up - it was less hassle, and they work incredibly well.

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2009 Lamborghini Gallardo LP560-4
(2006 Z4 M Roadster - Mary's )
(2012 X6///M )