It's better edge holding by a pretty good margin but I don't have any percentages. Performance wise it's more like D2 than 440C and the XHP is basically a cross between the two. The XHP can be considered either a high carbon 440C or a true stainless D2.
Carpenter has a powdered 440C that is called CTS-40CP.
The 40CP holds an edge much better than some conventional 440C from another supplier. I did some comparing of that with two identical blades and when cutting heavy cardboard and rope the 40CP held a keener edge by a pretty good margin. I've got some regular 440C from Carpenter as well but haven't used any of it yet to see how it compares.
Ask Less George about the XHP, he has used it much more than I have and he had a knife being passed around and was incredible what was being done with it. I've only had enough of the steel to fall in love with it.
XHP or 40CP is my new favorite steel. I just haven't figured out which one.