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Personally, I don't like the inkjet metallics very much, but I have profiled them for people that do like them. Even though they say the metallics can't be profiled, there didn't seem to be any particular problem making profiles, and test prints maintained a very neutral gray ramp and an overall they looked very good.
Since we're talking about inkjet metallics here and not offset, and there are so few people using a true CMYK RIP for inkjets, no one needs to concern themselves with anything regarding cyan, magenta, yellow and black, color separations or the like. You just treat the metallics like any other paper and send RGB data through the printer driver, which, in turn will do the final black box separation to however many inks your printer happens to use.
Unless there has been a new metallic introduced recently, the common wisdom is that all inkjet metallics come from one manufacturer - Mitsubishi, so you're getting the same product no matter what it says on the packaging.
Metallics for RA-4 process are another story altogether. They seem to be much more "metallic" in their underlying reflectance than the ink jets and are also more difficult to profile - at least when I did run metallic profiles years ago, but if you want the ultimate in glassy surface, those chemical prints still offer a surface unmatched by any inkjet media.