Swagfu, I understand, stuffing the wheel wells.  I personally don't think going with two different size tires on the 08 xB will do much for it in any way including looks.  When you start playing with different size tires on the front vs rear you also change the handling.  The old muscle cars were heavy in the front and rear wheel drive and the weight transfered to the rear under hard acceleration.  The bigger tires were helpful there and did look good.  On a front wheel drive car I think it looks silly.  The rear is already jacked up from the factory with a lot of space around the wheel.  Another problem with running two different size tires is it prevents you from rotating your tires from front to rear every 5,000 miles as recommended.  I don't know where you live but if you get much rain and want a great handling tire that is quieter and has a long life I would go with the Michelin Hydroedge.  This is the tire I will be getting in a 225/60-16.  It is 26.5" tall compared to the stock Bridgestone with a hight of 24.9" or 25".  This will give you 3/4" more tire in the wheel well and raise the car 3/4".  They also make the tire in a 235/60-16 with a hight of 26.9" tall but I personally think this is too much tire for the car.  A good source of tires is the Tire Rack,  http://www.tirerack.com/index.jsp  they are one of our sponsors too.  You can look at their tires and read the reviews and compare the different tires all on their website.  The front of the 08 xB is too low for me, 5.1" when new.  I am always hitting the front of my car on those cement parking lot stops.  I also drive in Mexico and on other poor roads and on the back roads where the extra clearance would help a lot.  As far as the speed error I don't know if there is a way to recalculate it or not.  I have the auto transmission and with only a few hundred miles on stock tires my speed was one MPH fast.  I happen to drive through four of those check your speed radar units using cruse control and every time it was one MPH fast.  Going with a little taller tire will change the speedo error in the opposite direction.  You will now be going faster than indicated by 1-2 MPH with new tires.   Just my thoughts.

Al