Thanks for the detailed reply Andrew.
For now we have completely disabled HDR in the project (even though it would be great to use post processing effects on Metal-level devices), since there is no straightforward way to enable or disable HDR or the tonemapper per device.
Disabling the tonemapper would have been a nice optimization (as we saw an approximately 10ms saving) for devices not using post processing effects, but running the game in HDR at all is too costly on ES2.0 in the end.
This is probably a diminishing problem for Epic as new ES3.1/Metal devices come on the market, but currently there is no ideal way to target both high end (HDR) devices and low end (LDR) devices with a single binary on mobile. It really is all or nothing, unless your game has extremely low rendering requirements.
Not the end of the world, but it means we can’t really show off the power or beauty of the UE4 on iOS, since Metal devices are required to use LDR if we also want to support low end devices.