I know this info is wide spread, but according to the service manuals the DACs were pretty sufficient with 16Bit and 48khz, at least on the later 4Op and newer DX7 Models.
It’s rather the internal resolution of the synth: To make the synth affordable and polyphonic Yamaha had to use some tricks, like rolling out the calculation by using lookup tables and working with the similar Phase Modulation instead.