if they maintain polygamy, they could perhaps have a notable population increase within a couple generations. (I think.)
[looking now for anything on Mormons and slavery... this is interesting... kind of interesting... hm... hm!... )
I tried to see if there was even a remote potential for MORMON DRAKA running a very large chunk of Mexico, but, I couldn't find anything to allow that leap to be made. Bottom line is that, from the bits that I read, Mormons weren't very much into slavery.
You are correct in assuming that Mormons weren't into slavery; in fact, in Missouri (and Illinois), part of the fear about the Mormons were that they tended to vote in one huge block, and were primarily Easterners who weren't too keen towards slavery. Not all of them were abolitionists (and in fact a few Southern Mormons were small-time slaveholders), and Joseph Smith spoke both against and for abolition, but the perception was that the Mormons would free the slaves.
As for polygamy, don't be so sure; it was mainly the leadership that practiced polygamy, maybe 10% (at the highest estimate) of Mormon males had more than one wife. Polygamy would not improve the numbers past a small blip in the statistics.
Now missionary work, that was the real population increaser. In the early years, literally thousands of converts came to America, and went straight to first Ohio/Missouri/Illinois, and then Utah. In fact, there was a sort of Church welfare program to fund the converted immigrants.
I'm thinking that I could have a PoD being a few of the splinter churches coming back to the main one when it gets to Utah, but none of them had more than a few hundred members, so that wouldn't affect the numbers terribly much.
The PoD could also have something to do with the Mexican-American War, but I don't know what. I don't want a Mexican win in the war, since it is fairly implausible and also becoming an AH cliche.
