I think it would be a fine caliber in the XP. The efficiency of the shorter cartridges has been around for a while (22,6,6.5 BR). For me the WSM and WSSM cartridges work great. with the shorter barrels,I know I will not get the full velocity of the short mag case but I am getting the velocities of what a standard case would be from a 22" to 26" barreled rifle, ie: my 300 WSM
HS velocities are what a "06" improved would be from a 22 " barrel.
long story short is that you will get higher velocities than the 308 based case and less than the short mags advertised velocities
go for it and keep us updated on your new caliber. looking forward to the info on the 6.5 on a WSSM as well.