@rossandmaureen also DM'ed me. I'm going to copy my reply here:
"I'm not sure how you came to the conclusion that it's not suitable for Nitrox. There is a warning that applies only to EU countries that says it is not, but that's because of an EU regulation that requires regulators used for Nitrox to have a different type of DIN fitting on the first stage. By the way, this regulation is generally ignored even in Europe outside of some parts of France and Germany.
The manual specifically states Nitrox up to 40% is fine for the rest of the world.
Here's the warnings from the Proton Metal manual that came with the 12 or 32 first stage.
Similar warnings are found in every manual for every regulator that is sold both inside and outside the EU since 2000 (the regs with the EU Nitrox fitting are generally sold only within the EU).
The Prestige 15X is a fine reg, but IMO the Proton Metal seconds are superior to the Prestige seconds. What first stage do you have? The 15 may have a slight edge in performance over the 12, but not over the 22, 32 or 16. I thinks that covers all the possibilities.
FWIW, I have hundreds of dives using Nitrox with both the plastic and metal Protons without any problems."