1. P-Audio specify the true mathematical Xmax, you can often go higher than this by 20-30% without ill effect providing you are sensible. Some manufacturers used a different calculation which exagerrates the Xmax.
2. How much excursion you need also depends on the frequency range you are operating at, some cab designs control excursion better.
3. Just because the max power is 1200W doesnt mean you have to use all of it, with 1200W total power, you could happily tick along at 800W without fear of power compression issues, or danger of blowing the driver, sometimes keeping a bit in reserve is a good idea.