Tire size bias and winter tires

[nd750]

Here's the situation: I have some 265/35 R20 snowtires from my S8. They're less than 2% difference in diameter than the stock 750xi size, I know that should be kept less than 5%.

How important is it to maintain the tire size bias in the 750xi? (my only other experience with that is Porsche, but 2wd and mid engine) I know diameter is more critical, how about widths?

I can't get stock sizes 245 front 275 rear in a matching set of snows, but I can get Blizzak rears, and Sottozero fronts. I'm told that its "very bad" to mix the tires, but logic has me doubting because there's a differential friction amount due to "footprint" of the different widths to begin with. Additionally, in the winter all 4 wheels are likely experiencing different friction levels due to the surface from time to time.

Finally, I'm told its just fine to have 245's front and rear (on 19" wheels), and Alpina has set up 245's front and rear for snows, however, its a rear drive car.

I understand that with a 265 front width, I have somewhat less traction due to more flotation, but I'm trying to use my S8's tires (only have 2K miles on them) before spending $1500 on a new set.

Am I reading into this too much? Is X Drive real picky about these things?

Thanks for the help.

[nd750]

Just to answer my own questions:

1) my race shop guru confirmed that width isn't so important as the diameter - width will affect traction, of course, and over/understeer tendency.

2) dealer installed my 265/35 R20 Sottozeros on the car, and they fit just fine (there was some thought that the shock tower in front may be the reason BMW put 245's up there)