Symmetry of both photodiodes at higher temperatures will become worse! I never used IL300 at +70°C, only at about room temperature
OK, if you are designing for operation in air-conditioned environments, you may be able to get it 'fair'. My equipment has to work at 0-75 degrees and there are cases where it does not work because of too high temperature when the bus is parked in the California desert between the morning and evening rush hour. This resolves itself a while after the bus is started when the AC seeps into the cavity where the sign is mounted and bring the cavity temp to abt 50 C. My reaction was to a post that stated that with an analog optocoupler you would improve precision and adding an analog optocoupler, how ever close to linear, will definitely not improve precision.

Erik