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.
In person early voting has been in place in Texas for several election cycles so that is my experience. I don’t know when other states placed it into effect. But my point is based upon voting patterns there are normally less voters in non presidential elections. The numbers may actually favor the Republicans more if it could be compared to an off year. In the end it will be what it will be regardless of the cycle.