I use an ATV 4K for my 55” VT65 plasma, as I decided to quit purchasing discs now that Apple are upgrading to 4K for free. However, I couldn’t shake the nagging feeling that I’m giving up some picture quality now for gains in the future, when I eventually retire the plasma for a 4K TV.
Given how close to UHD Blu-Ray iTunes 4K material is, I decided to purchase a HD Fury Linker. This enables me to access the superior bit-rate of the 4K titles by making the ATV think that it is connected to a fully-capable 4K DV/HDR UHD TV. I set the ATV to 4K SDR (the ATV does a really good job of DV/HDR conversion to SDR imo; I’m used to a calibrated image and the converted picture looks natural) and the Linker downscales to 1080p (and sorts out EDID/HDCP).
The resulting picture is very close, if not the same as, Blu-Ray.