Another interesting question is whether/when Texas could pass California. Using a straight line model, this could happen sometime in the 2050's. I'm not sure straight line growth is appropriate here because inland Texas may well hit its ecological capacity long before then, but let's say it's possible. Could we be looking at this apportionment come 2050?
Seems unlikely. It's pretty much impossible for Texas's growth to go any faster, and keeping up its current pace for more than another decade or so will be very difficult. Additionally, Texas's economic strength is based entirely on the strength of the energy sector, particularly oil. In the long-run, it's doubtful the current oil boom (either in production/extraction or in demand) can continue more than another decade. The environmental stresses you mention will also start to come into play eventually, but that's only really important for isolated outposts like Midland/Odessa and Lubbock. There's also the immigration factor: Many border towns are growing quickly due to immigration, but it's hard to say how immigration will continue over the next 40 years. I would have to assume immigration will decrease over time, especially given the poor economic state South Texas is in.
tl;dr: Texas will continue to grow quickly for about a decade, then slow down, primarily for economic reasons.
In addition, I doubt that the exponential decline of the Midwest and Northeast will continue; it's already starting to slow. Once the Sunbelt starts to shed people thanks to a combination of global warming and lack of jobs, they'll head back to Yankeedom.
