The AMD beta mining version has LESS support for older cards (not a big deal if you don't mix them with new stuff) than the current 17.50 release, and doesn't support underclock or undervolt at ALL as far as I can tell.
It also has widely reported issues with "doesn't work with VEGA", though there appear to be possible work arounds for those issues in some cases.
rxOC comes with block chain drivers(missed that in first post of this thread) version 17.10-429170. Not sure there is much hope for older card, I believe on my regular non rxOC install AMDGPU-Pro worked with my older 7850 and R9-270X cards. Looks like folks have had success getting VEGA working by installing latest kernels. As you know, the state of AMD drivers in Linux is not good.
https://www.reddit.com/r/linux_gaming/comments/7sz8fc/rx_vega_and_linux_gaming/