According to Apple Specs, ATV sends/supports 5.1, 7.1 or Atmos, so if it's an Atmos capable device AVR, then you'd get the full discrete 7.1 portion which would have the .4 overhead "down" mixed into the 7.1 as Atmos is embedded into the E-AC-3 stream but not an AC-3 stream.
Audio Formats
HE-AAC (V1), AAC (up to 320 Kbps), protected AAC (from iTunes Store), MP3 (up to 320 Kbps), MP3 VBR, Apple Lossless, FLAC, AIFF, and WAV; AC-3 (Dolby Digital 5.1), E-AC-3 (Dolby Digital Plus 7.1 surround sound), and Dolby Atmos