Sorry for the late reply.
Pretty much everything I would say has been answered. Personally, My favorite long range hunting chambering for the XPs are the 6.5mm WSM, 7mm-270 WSM and the standard 7mm WSM
as well. The last two are basically identical with the 7mm-270 WSM offering the ability to use the better quality Norma cases with really no more bench work needed or fireforming required.
The 7mm Dakota is a beast in the XP. Only reason I do not have this on my list of favorites is because I do not or have not owned one personally! Power wise, the Dakota is about the tops in my opinion in the XP handguns.
That said, a quality bullet in any of the mentioned rounds will get you great performance at long range.
One thing to remember is that your in a different velocity catagory then compared to rifles of these same calibers. As such, we need a light jacketed bullet to ensure expansion at long range. Also, because the velocity is starting out lower then normal, we want a very high BC bullet to retain as much velocity and energy as possible.
Whoever you have build your XP, make sure they have built these large chambered XPs before because there are some special considerations when stocking one of these hand cannons. If you do not address these issues, your stock will fail relatively quickly. First big XP I had was a 338 WSM. Great round, hell on big game and stocks!!! Broke the laminated wood stock three times until I got fed up an installed enough steel in that grip to handle the recoil.
Have it built right with the right componants and you will be very happy.