Coming back to this with some new research.
Perhaps; however, please keep in mind that population growth in Mexico appears to have really took off in the 20th century in our TL. In contrast, in this TL, population growth in Mexico could both take off sooner and end sooner--thus still resulting in approximately the same population as in our TL.
Reflecting on this, I do indeed believe there would be a jump as Mexico developed after joining the United States, but it wouldn't be as big as what was experienced as an independent nation in the late 20th Century. For example,
in the 50 year period between 1854 (Post-Gadsden Purchase) and 1904, the Mexican population grew from 7,859,600 to 14,209,000; for comparison, between 1950 and 2000, the Mexican shot up from 26,282,000 to 100,349,800. That the latter growth rate was due to the advances in medicine and food production (Green Revolution) seems obvious, and we can safely conclude such wouldn't happen ATL. So what do I propose would happen? Let's say that Mexico gets developed from 1848 onward to 1920, and thereafter converges in demographic trends with the rest of the United States as it reaches the same standard of living as their counterparts. Given such, I'd think you'd be more likely see 80 to 100 Million Mexicans, instead of the roughly 160 Million that there are today (Mexico+American diaspora).
Also, wouldn't it still get some good rail links in this TL and possible still get the movie industry to come there?
Essentially, what happened IOTL that got them said rail links was the existing Central Pacific monopoly trying to handicap the threat to it posed by the development of the San Diego route. As well, as the article notes, not only would the San Diego route probably eliminate Los Angeles as a major city, it'd likely do the same to San Francisco once the Gold Rush ends:
It was at this moment that Stanford, in an interview published in the San Francisco Chronicle, first set forth publicly the plan to push the Southern Pacific from Yuma across into Texas. In concluding his interview he said: "The people of San Francisco will never appreciate how great a danger menaced them . . . Had Tom Scott built his road to the Pacific he would have taken from us our best prospective traffic and carried it East . . . He would have given San Francisco a blow from which she would never have recovered."
As to why the Southern route wasn't built first, the answer lies in the sectional disputes leading up to the Civil War. The North, quite obviously, favored the OTL route since it geographically favored them, while the South preferred the, well, Southern Route. The Southern Route was superior in terrain, as it didn't cut through the Rockies, and ultimately the momentum for it is what led to the Gadsden Purchase in 1854. The Civil War wrecked these plans, however, and by the time such could occur after the conflict the Central Pacific monopoly had emerged.
It would be fascinating to see a more Latino US.
You'd probably see a lot of minor things, like Salsa emerging far earlier as a favored condiment and Spanish loan words entering into mass usage in the rest of the United States. Earlier Mexican communities in American cities would also lead to earlier introduction of Mexican cuisine, which could lead to more regional variations on the same format as "TexMex" food. The biggest one, in my estimation, might be the abandoning of the "One Drop Rule" in favor of the Latin American Concept of
Branciemento. The media at the time propagated the idea of romance between American men and Mexican women, even going as far
as to write poetry on such. These sentiments did not stop at rhetoric, however,
as such inter-marriages were actually common in the parts of the Mexican cession that had existing, sufficiently large populations and were, apparently, considered respectable. Should such a concept gain national acceptance, it could over time come to be applied to other racial groups, which would be a change from IOTL; not a less racist America, mind you, but a different outlook all the same.