I'll take a stab at this one. You should not have a problem going with the following:
The slight difference in the sidewall size will compensate for the 20" as opposed to your stock 19". However, going with 275 and 295 front/back, you may have offset issues. Also, with the wider tires your tire/wheel will be quite large and may look weird in relation to the car's wheel wheel even if you get past the fitment issues. You may want to consult with your local tire shop to confirm. You can try tirerack.com as their website is very informative.
2013 BMW X5 35i
Alpine White | Cinnamon Brown | M Sport
2013 BMW 550i
Titanium Silver | Black | M Sport
Last edited by Decboy; 09-18-2012 at 10:06 AM.