If Romulans are really as powerful politically as we are repeated told in the different stories, then I would image they would have to have quite a few colonies out there with a lot of Romulans on them. Not to mention all of the Romulan ships that were probably out and about when the Hobus event happened. At this point I doubt they are in any danger as a species, but the Empire as a political entity is probably a different story. Since the supernova destroyed Romulus and it's star system, then that means the Empire has lost it's homeworld and the majority of it's political, and military leaders. So I have a feeling that whatever is left of the Empire as political and military entity is probably a lot weaker than what it was before.
I think the effect on the Empire might be akin to that of destroying the Eastern Seaboard of the United States but sparing the remainder of the country. The large majority of the Empire's population, productive capacity, and military remains intact, and when a new administration is installed it will resume its great power status, but for the time being it's going to be preoccupied with cultivating (rather, rebuilding) its own garden.