Clemson and SC had about the same average number of viewers last season and Clemson got a bump thanks to being in the CFP where the audiences were much higher than their regular season games and the ACCCG. Outside of those games, Clemson was only in the top 5 once, when they played UGA in the kickoff classic game. FSU was in the top 5 once, their game on August 24th against Georgia tech. I think that was the only game on that day.
That said, UNC nor UVA were ever in the top 5 of games watched. I think UNC and UVA bring more than football to the SEC. Both are established top tier academic schools and both are pretty well rounded in athletics outside of football. Most years, Clemson is a one trick pony in sports, same for FSU. Also, picking up UNC and UVA would give the SEC the entire Southeast as its footprint with all states contiguous to each other, the map would be very tidy.