Naruto learning to do Shunshin in base this late is pretty stupid, imo. Sasuke has been doing it since the beginning of Part 2, Kishi just forgot about Naruto. The only other time i remember him doing it is the first time he used KCM against Kisame.
Anyway, i actually think that their skills have been dulled. I mean... there's peace now. Naruto and Sasuke can handle mid-Kage level opponents with SM/MS alone. For them to need to resort to Rikudou Mode/RinneSharingan it would take an Ootsutsuki level threat, which there's no way they encountered, after Toneri. Since they have been living peacefully, i don't think they've improved much, if at all.