Bitrate is not going to tell you that DD+ is a better encode than DD, in fact, the bitrates might be lower for DD+ at the same number of channels, but it is a higher quality lossy encoding.
When you set the surround to "Best Available", the ATV decodes the highest quality audio, mixes system sounds, then re-encodes to LPCM up to 7.1 channels. For upcoming tvOS 12, Dolby Atmos will be implemented by decoding the DD+ Atmos audio, mixing system sounds, then re-encoding to DD+ Atmos...however, regular DD+ will still output as PCM, (as seen by current betas).
The TOSLINK/optical connection can support DTS (lossy) & DD+, but typically only has hardware to support DD 5.1, so going through your TV will limit your results, as the TV will take whatever you give it and output DD. [Optical is capable of much more than copper, however, development was pushed back by the copper industry, or else we'd have a new optical video+audio connection instead of HDMI today. (OWC made a 30 Meter (100 ft.) optical thunderbolt cable that had no speed degradation). Yes, one might argue fiber optics have a higher cost, but, that could be brought down if the industry had really gone for it and kept developing the tech at each end of the cable, and overturned the industry so they were making more optical than copper cables, the more mass produced, the lower the cost.] [Also, the internet is connected between continents via fiber optical cables.]
