My opinion is if you want .357, then use .357 brass. I don't see the need to stress 38SP brass.
Everglades recommend using jacketed bullets for load data, to a max velocity of 1200fps.
Hodgdon has 5.4 - 6.1 for the XTP. My opinion here is if you can get that amount of powder in a 38 case, why would you want to risk pressure spike into the realm of OUCH?
DON'T DO IT!