I've never found anything that matches the pure mirror gloss of fujiflex except for Ilfoflex, which is pretty much identical (and is a chemical based photo paper as well). The Ilford is substantially less expensive, and side by side I can't tell the difference.
The problem with the inkjet papers mentioned is while they provide some similarity, you still have inks sitting on top of paper and thus a loss of the pure mirror gloss surface. Many using fujiflex also facemount it to acrylic or lexan, and it lends itself to that very well. If you could facemount one of the mentioned products you may come very close to the same look, if you are laminating then it might be close as well (although I've never seen a laminate that matches the absolutely smooth surface as the flex itself or the facemount, and most laminates seem to have a look to them that isn't quite as nice),
Many inkjet products do not facemount or laminate well, I have no experience with the ones mentioned. some tend to delaminate over time as the ink separates from the paper base. I've also heard there are some issues with longevity with those products, but that may be when un-finished and used in other areas like POP advertising, and may have nothing to do with fading etc. (ernst would know more about that than I would.)
Good luck with your tests, let us know what you think when you are done.