The Dominion would clearly be defeated as an empire by the Borg. The Vorta and Jem'Hadar don't possess either the technology or the cleverness and independence of thought to be a match for the Borg. The Changelings would have to do the thinking for them, and I think they would rather hide from the Borg, instead.
In my view, the end result: the Founders send the Vorta and Jem'Hadar to their deaths or assimilation, and while using them as cannon fodder and a distraction, the Founders escape the Borg and remain undetectable by them.
Rubbish. The dominion in the Gamma quadrant has as many speices if not more than the federation. They would be pumping out tec like mad.
If the Wormhole had been open to the Dominion they would have crushed the Alpha quadrent with little effort.
As for assimilation the dominion would never let that happen, they would destroy there own planets before they let them be turned over to the borg. There ships would self destruct as soon as the first drone steps on board.
As for tatics the dominion have a limitless supply of ships. As soon as the borg adapt they would just fly there ships into the borg ships until they came out with a new weapon.
Look at how quick the dominion could pump out ships and Jemhadar useing just the the industry of a 3rd rate power like the cardassians! And in the Gamma quadrent they have a Empire that spans a good portion of it with centeries to build up ship buidling facilites.