Update from interaction with Italian EPSON support: completely useless and really disappointing! The first level support continued to say that Luster paper is not supported (first everywhere, then only in Italy, then again everywhere), even if we know that several users have luster in their driver, and the US documentation report it! They were unable to explain why, if the paper is not supported, I have an ICC profile on my Mac specifically for ET-8500 and Luster paper installed by their software. They refused to pass me to second level support, and added that according to their technical office the driver could contain options "non available or not compatible with a printer" (???)
I will try using the semigloss as suggested.
Thanks to all, and keep away from EPSON support.