For today that is impressive. 40 years ago, when there were less channels, that wouldn't be as good. The orignal series in 1974-75 had an average rating of 6% and that was considered bad. (Its third rerun in 1978, during Yamato Fever was 13.8%)
Yamato 2 (Comet Empire series) had an average rating of 22.9% on its first run in 1978-79. That series was awesome. Also that was the high point of Yamato Fever.
Will lightning strike twice? It is starting to look like it. Will the Yamato Fever return? Maybe.
Just so you know the highest rated anime in Japan is usually Sazae-san, which has been on forever. It is getting in the 10-20% range these days.