here's my setup mi box > wireless 5ghz > asus rt-n66u (not AC) > NAS wired to router .
whenever i experience buffering from a huge file 8GB+ i switch over to 2.4ghz. direct stream from the internet (youtube, iflix, netflix, crackle, and similar) has no issues regardless if 2.4 or 5ghz.
the main player is kodi, latest version. haven't tried playing it on vlc or mx player.
your buffering issue can come from various reasons - router, player (and settings), file size, interference if wireless, etc.