This is true also, and that boll wevill (or whatever bug it was) that devestated the south in the late 1800's will probably still take place.
All that being said, cotton will always be big in Texas, I'm a 5 minutes drive from an endless plain of cotton fields along the Brazos. It was even bigger in the mid 1800's. If anything the early loss of the Rio Grande Valley and bits of West Texas will make east-central cotton even more important.
That is true, but without modern fertilizer that wouldn't be possible. And most Texas cotton is pretty much limited to the river bottoms within about 100 miles of the coast. A lot of the current African American population is actually descended from slaves who were imported in the first couple of years of the Civil War to safeguard them from Union campaigns (and to make it harder for them to run off). There were sizeable numbers of slaves in Texas prior to that, but the number took a big jump in 1861-62.
The other big issue for Texas is the nearly 40 year war between Texas and the Comanches that wasn't really finished until the 1870s and began during the 1830s. That will tend to keep Texas a well armed society and the Texas Rangers busy for nearly the entire period. Luckily for Texas though, the Republic of Mexico had bigger problems then Texas for the 1837-1843 period, including more civil wars, a Spanish invasion (that failed) and political divisions.
Texas could possibly have gotten larger if the expedition to take Santa Fe had been better planned, led and luckier, although how that would have benefited Texas until the Comanches were crushed escapes me. Communications would have been unreliable, and safer along the Santa Fe Trail (which avoids a lot of Comanche territory as it comes to Santa Fe from the northwest), and of course that trail connects St Louis to Santa Fe, providing no benefit to Texas at all.
I can't see Texas having a real shot at New Mexico or Arizona and especially not California unless the Civil War occurs, Texas stays out of it, the Union loses, and it benefits from the Union (or the CSA or British) sinking some capital into Texas allowing construction of the real life Southern Pacific Railroad line from San Antonio to southern California.
I think California is certainly doomed to become US controlled after 1849 (or whenever an alternate history gold rush occurs), and certainly so once silver is found in Nevada (late 1850s). The sheer weight of numbers of American settlers would sweep aside the rather small number of Hispanic Californians (small compared to the tidal wave of American settlers). Some incident would set off a US/Mexican War, and the Americans can conquer California fairly easily enough even without an overland march (as they essentially did so after all with a relatively small force).
Texas has no such option (no significant seapower other then the Texas Navy, which was small although fairly effective for its size during its brief existance).