I wish I could find more updated info about commuting into Travis County, but 2013 seems to be the last year for data. According to this map, Burnet and Blanco Counties are in the same 20-30% range as Caldwell County which is part of the Metro. I would think that both counties should be close to being added to the Metro by now. Together they would add about 58K to the Metro population which is about the same as the population growth in an average year for the Austin Metro.
https://www.texastribune.org/2015/08...-commute-work/