If the Borg saw them worthy of perfection they would go after the Jem'Hadar, Vorta, Changelings and any other member of the Dominion they wanted.
A Borg-Dominion War would be a devastating event though, very epic to see or read about, but I doubt there would be little left of either once the dust settled--no matter who actually 'won'.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)