The way San Antonio is growing, I doubt metro Austin will ever surpass metro San Antonio. It will never surpass S.A. city proper, thats for sure. San Antonio is almost twice the size, city pop. wise. The metros may come close by 2040. Cities don't keep up the same pace over the decades. Bexar was the fastest growing county in raw numbers between 2000-2007 in Texas. That should give you an ideal of San Antonio growth.
50 largest metros 2020
http://proximityone.com/msa06rnk.htm
1. New York-Northen NJ ...............................................19, 489,766
2. Los Angeles-Long Beach-Santa-Ana ...........................13,415,189
3. Chicago........................................... ............ 10,255,361
4. Dallas-Ft.Worth-Arlington ......................................... 8,022,433
5. Houston-Sugarland-Baytown..................................... 7,419,345
6. Atlanta-Sandy Springs-Marrieta.................................. 7,291,964
7. Washington-DC-Arlington-Alexandria VA...................... 6,120,987
8. Philadephia-Wilmington........................................ ...... 6,095,446
9. Phoenix-Mesa-Scottsdale........................................ .. 6,088,369
10. Miami-Ft Lauderdale-Miami Beach.............................. 6,023,996
11. Riverside-San Bernadino-Ontario............................... 5,735,261
12. Boston-Cambridge-Quincy....................................... 4,557,239
13. Detroit-Warren-Livona............................................ 4,409,268
14. San Francisco-Oakland-Freemont............................. 4,314,064
15. Seattle-Tacoma-Everett........................................ 3,825,817
16. Minneapolis-St.Paul-Bloomington.............................. 3,635,478
17. Tampa-St.Petersburg-Clearwater............................. 3,390,335
18. San Diego-Carlsbad-San Marcos.............................. 3,161,815
19. St.Louis................................... 2,997,615
20. Denver-Aurora.................................... 2,982,418
21. Baltimore-Towson................................... 2,871,894
22. Orlando-Kissimme.................................... 2,802,272
23. Las Vegas-Paradise.................................... 2,731,563
24. Portland-Vancouver...................................... 2,626,123
25. San Antonio..................................... 2,582,903
26. Sacramento-Arden-Arcade-Roseville......................... 2,555,024
27. Cincinatti-Middletown ............................. 2,366,078
28. Charlotte-Gastonia-Concord,NC-SC...................... 2,335,300
29. Austin-San Marcos-Round Rock....................... 2,278,323
30. Kansas City,MO......................... 2,260,809
31. Pittsburgh......................... 2,203,434
32. Indianapolis-Carmel........................ 2,022,063
33. Columbus,Oh........................ 2,018,391
34. San Jose-Sunnyvale-Santa Clara............................. 1,994,421
35. Cleveland-Elyria-Mentor............................. 1,972,501
36. Nashville-Davidson-Murfreesboro............................ 1,961,485
37. Virgina Beach Norfolk-Newport............................. 1,808,892
38. Jacksonville ............................. 1,658,033
39. Milwaukee-Waukesha-West Allis ........................ 1,613,544
40. Providence-Fall River......................... 1,576,328
41. Raleigh-Cary.............................................. ..........1,560,659
42. Richmond, VA................................................ .......1,458,047
43. Memphis........................................... ...................1,438,103
44. Oklahoma City.............................................. ........1,395,135
45. Louisville-Jefferson County.....................................1,383,3 81
46. Salt Lake City ........................ 1,370,089
47. Hartford-West Hartford......................... 1,252,127
48. Birmingham-Hoover............................................ ....1,230,570
49. Tucson ........................ 1,217,465
50. New Orleans........................................... ...............1,097,507