The Federation Alliance only won because the main bulk of the Dominion fleet was wiped out in that wormhole and that the Feds got control of the wormhole back. (the Cards switching sides helped too)
A full strength Dominion empire would give The Borg a money but there resources are better so eventually The Borg would probably win though the founder most likely cannot be assimilated.
I always wanted to see the entire Borg species get thrown into a war with the major powers of the entire Galaxy to null the Borg threat once and for all. Just imagine...
Feds, Roms, Klingons, Cards, Breen, Thoilans, Kazon, Vidiians and many more all teaming up to kick some Borg ass.