I think the comparison should be made over 2006 rather than 2008 as that was a presidential election year. There are larger turnouts during presidential years as opposed to non presidential years. The same trend may be present in those years but I think it would be meaningful.
Texas data for the top 15 counties indicates that the total will be up over 2006 but will fall significantly short of the 2008 number. That trend is typical for years back in Texas that I have looked at.
Example Texas has just over 13% turnout of early voting/mail in ballots during non presidential years 2006 and 2002. Whereas it had over 42% in 2008.
2008 is the best data we have because early voting really didn’t take hold until 2008. It’s going to take a few election cycles to really pin it down so we can compare apples to apples.