Having a read on a forum someone came up with an interesting concept.
Printing from the outside inwards why cant we set the first layer of the shell to be say 0.1mm and do two passes then for the rest of the inner layer do them at 0.2. this would create a high definition surface and allow you to still print the rest of the item fast.
The setting could be done as a division of your internal layers. say if you set your internal layer to 0.1 then your shell layer would be 0.05 etc..
Im sure with some creativity you could even graduate the passes from the outside inwards with the say the first pass of the shell being 0.05 and built up 2 layers, then the next being 0.1 and build up 1 layer, then go back to the shell and built up another 2 0.05 layers and in to the next pass of the shell and build up another 0.1layer then the rest to the center at 0.2...
Doing it this way would create a huge amount of detail on the outside.
Re: Variable Layer Depth
You can already do this for the most part by setting the software to "combine infill every 2 layers" on the Infill tab. For example, if you had a 0.1mm layer height, that would print two fine 0.1mm outlines and then one thick 0.2mm infill layer. I use it all the time. It give a big speed increase while still having good detail on the outside.
