The XM-L has pretty much replaced the P7. You can easily drive it at 3A and get up to 1000 lumens. The SST-90 can give you more output (2500 lumens) at up to 9A. Some people are using multiple XM-L's in lights to get tons of output.
I've also over-driven the XM-L and as long as you keep really good heat sinking, they're safe up to about 4 amps using a well heatsinked star mounted XM-L. Continuous use at 5 amps or more will burn the phosphor around the traces though. But my best guess would be if you get 1000 lumens at 3 amps you might get 1200 at 4 amps and corroborated using a ceiling bounce and light meter, that's extrapolating from Cree's data sheet for the T6 bin too. It's slightly brighter if you compare them side by side from 3 amps to 4 amps so my guess 1200 lumen guess would be in the ballpark.
But in all respects but one the XM-L bests the P7. The one caveat is color, everyone here will tell you that in a reflectored light the XM-L will make a noticeably yellow hotspot with a blueish spill. It has a weird color distribution. The P7 lights I've had always had a pleasing creamy white tint that was even across the beam.