I am jumping in a bit late to this party however I would like to add my experience with XTP performance on game. I have had 3 instances where an XTP failed to open on deer.
All shots were a 300gr XTP (44 mag) pushed at about 1250 fps.
Shots ranged from 2 yards to 50 yards.

The 50 yard shot went through the spine, just above the front shoulder. The bullet turned around and was found base forward just under the hide on the opposite side.


A followup coup de gras from 2 yards went through the heart and was also found backwards under the hide on the opposite side. The bullet looked similar to the first one but with more pronounced slumping.

On a side note, I guess I can join jamesfromjersey's backwards bullet club

I also had a 20 yard shot where the hit was right in the crease behind the front leg. The bullet penciled through, getting both lungs in the process, resulting in a ~200 yard tracking job. The lungs showed a nice clean caliber sized hole in and out. It didn't look like the bullet opened up at all.

Both animals were found and I made what I believe to be decent shots.
I think that I will go back to cast (WFN) style bullets or maybe give the Barnes XPBs a try.