I don't see a single Romulan system as being any kind of threat to the Federation.
The feeling of the Romulan encounters in TOS is one of controlled fear: they're a serious threat, even if the BAT-BOP was no match for the Enterprise, a war with them would be protracted and costly... and any miss-step on Enterprise's part would cause such a war. That's not the attitude Kirk would project for a one system species, as such a species simply would not have the resources or manpower to be a threat.
I would make similar arguments about a non-FTL Romulan Empire.
And I don't see the Federation sitting back and doing nothing while the Romulans are effectively occupied by the Klingons.