I was curious so I looked these up:
Burnet County
Population Estimate, July 1, 2016: 46,243 (+8.3% since 2010)
Source
Blanco County
Population Estimate, July 1, 2016: 11,392 (+8.5% since 2010)
Source
Crazy how similar their growth percentages are. It seems that if they continue to grow at these rates, 290 or 71 (or both) will need to be upgraded to an expressway in the near future to accommodate local growth as well as commuter traffic. This is especially true for 71 all the way to 281. I think 290 needs to be an expressway at least to/through Dripping Springs.
As for adding them to the metro, it only makes sense, especially considering that Caldwell County has similar commuter patterns. For what it's worth, here's the info on Caldwell County (nearly identical to Burnet County):
Caldwell County
Population Estimate, July 1, 2016: 41,161 (+8.2% since 2010)
Source