If you increase the rim size, you need to have correspondingly smaller sidewalls to keep the outer diameter of the tire the same. Otherwise, your speedometer will be wrong (I assume it would affect the odometer as well then). I would think it also affects any other sensors that are speed based (traction control, stability control etc.?). Maybe those operate more on comparing the speeds of the tires against each other though.
As for performance, physics says less weight will make a difference. Though I have to wonder if it is enough to actually be noticeable on a daily driver like our xB's. I'm sure someone on here has done research about it though.
I went from 16" stock steelies to 17" alloys. The wheel/tire combo was a little lighter. Mostly I noticed a slightly rougher ride from the shorter/stiffer sidewalls of the 17's and better cornering from stickier summer tires vs. stock all-season tires.