The consequences would have been unlikely to lead to outright war, but it is very likely that Sisko and Garak would have been given over to the Romulans for trial. Dr McCoy and Captain Kirk were similarly given to the Klingons for trial and imprisonment during the film Star Trek VI: The Undiscovered Country.
Cardassia certainly can't afford a war with Romulus at this point, and the Federation, while better off than the Cardassians, still came out of the war worse off than the Romulan Star Empire. All told, the Romulans seem to have come out of the Dominion War as the dominant power in the region, with the Federation scrambling to catch up, as evidenced by Federation willingness to acquiesce to Son'aa demands in Star Trek: Insurrection.
As events occurred, the Dominion War was followed in the Romulan Empire by a military coup and a barely-averted war with the Federation anyway (seen in Star Trek: Nemesis) so the discovery of Sisko and Garak's actions may not necessarily have led to anything different in this case. If Romulus, an aggressive, expansionist power which already had friction with the Federation, didn't feel capable of starting a war against the Federation following the Dominion War in the normal time line, it is unlikely revelations about Federation actions during the war would push them over the edge.