I've been talking with FrSky about this and the problem (as I suspected) is not to do with resolution but to jitter caused by the need to run the RF frame rate asynchronous to the PPM data stream.
I'll do a full article to explain this but suffice to say it doesn't affect resolution. What's happening is that, at some rates of stick movement, the servo will move in slightly irregular steps (caused by occasional oversampling).
With a standard servo this is not really discernible because a standard servo stops driving the motor shortly after each pulse arrives however, digital servos continue to move to the commanded position because they effectively memorize the pulse.
This means a standard servo effectively integrates (smooths) the pulses but a digital one responds to each individual pulse and, because there's an occasional oversample, the movement has some jitter associated with it.
Does it matter in the air?
I would wager that it wouldn't but I'd be keen to hear from anyone experiencing this jitter as to whether they can discern the effect in the air.
Can FrSky fix it?
I'm talking to them at the moment and will be making some suggestions. One is to increase the level of oversampling so that the amount of jitter is markedly decreased.
I've been flying hardcore 3D with big surfaces, big throws and digital servos for several months now using the FrSky system and haven't noticed any problems. I'll be reviewing the telemetry system shortly and will look to make sure the jitter isn't greater in this product.