P2 is an output bin. It's the same regardless of the emitter type. However, it's only available in the XHP70.2. The older version tops out at N4.
XHP70 datasheet:
http://www.cree.com/led-components/media/documents/ds-XHP70.pdf
XHP70.2 datasheet:
http://www.cree.com/led-components/media/documents/ds-XHP702_1.pdf
From those, using the Easywhite 5000K 70 CRI data at 1050mA and 12V, the best XHP70 bin output is 1710 lumens (N4), versus 1830 lumens for the XHP70.2 (P2), so an extra 120 lumens or 7% output gain.
Because the latter also claims slightly less efficiency loss at the maximum 2400mA current, the outputs there should be about 3250 lumens and 3550 lumens. That's an extra 300 lumens or 9% gain.
Cree is also claiming a pretty significant reduction in forward voltage for the newer emitter - about 7% at max current. If true, then that combined with the increased output in that new bin means as much as a 17% improvement in efficacy for the best of the 70.2 line.