Just because you can doesn't always mean you should
The yellow is pigmented for 20-50 microns, so 15 is out of the limits that we have determined for this resin. You can still slice that low, but that is mainly a feature used to give some flexibility to the people who like to test different resins or resin combinations.
Like akgold said, 50xy20z is just going to take longer with no real tangible improvement. If you are doing 30xy20z (versus 30xy30z), you might see a small amount of improvement, but it certainly won't be a 33% improvement, and prints will still be taking longer and therefore using up the PDMS faster.
From my experience, 30xy 25z gives about the best detail and quality.