I personally think that Russia would not escalate the war if the US bombed the the Roki Tunnel ie you are not see Russia declare war on the US and go on total war footing with a massive push on Georgia and Eastern Europe.
If they had enough troops already in Georgia they may have tried to keep fighting. If not they would have called a ceasefire with a probable return to the status quo. Also depends on if the US with or without NATO establishes a no fly zone as this would leave Russia without land re-enforcements and no air superiority. Could Russia ship in troops quickly? though I guess that could be blocked too if the US is determined. Overall I think American influence/prestige especially among former eastern bloc countries would have increased. Also Russia may start spending even more on its military after the humiliation.