I just bought a hardened steel 0.4 mm nozzle for my Ultimaker 2+ for $25 from www.printed
solid.com and I am very happy with it. You can get a steel plated brass nozzle from Proto-pasta for $15. So those are two available proven options.
In vaporware somewhere i heard rumor of a gemstone nozzle of all the crazy things.
These tungsten nozzles would be a great option if they were available in 0.4 mm for the UM2+ and it's kin, but it was not obvious from the presentation what the nozzle parameters were. Maybe I just missed these details? Probably so.
Les