If a125gr JHP Gold Dot will not even start to expand in my J frame 38 special at 812 fps............

I doubt very much, that the heavier copper jacket on that XTP Hornady bullet will start to peal back, even at 900fps.

The 38 special JHP +P ammo has a reason to do 950fps, over the standard loads, for SD use.

I would try a minimum of 950fps as a starting load, to make sure the bullets work 100%, each time.
Noting worse than shooting a .44 FMJ bullet in a SD operation.

Good loading.