Most reputable manufacturers measure and match drivers (or match output by tweeking crossovers) for either speaker pairs or to match a model's reference (or output level) closely since any miss-match does affect imaging. I also believe that break in can increase a drivers output but may be only by 1 db or so.
Driver matching vs break in period
I installed a new mid range driver for my B&W 803 SII and noticed that the image shifted to the speaker with the older driver. I got out my radio shack SLP meter and found that the new mid range driver was about 2-3 DB lower than the old driver in the other speaker. I ended up reinstalling the old driver and putting the new one back in the box. The question is: Is this a matter of matching drivers, or is a matter of allowing the new driver to break in, or is a bad driver? Any thoughts?