If the Romulans stooped to using metagenic or thalaron weapons, they would win. The Federation would likely not use such weapons in a first strike. The Romulans could potentially use cloaked ships to decapitate core Federation worlds.
But in a straight up conventional war, the Romulans would at first gain the upper hand due to the cloaks, but if the Federation considered the treaty null and void, and start putting their own cloaks into service, or borrow the tech from the Klingons, thus eliminating the one true advantage they have over the Federation, then the fight is more even, maybe even tipping towards the Federation.
The Romulans are pretty homogenous whereas the Federation has many worlds and many species to draw expertise upon, and not just humans (IE the Vulcans, Andorians, etc), so I think the Federation has an edge there, tactically and strategically. Regardless, however, the Klingons would probably jump in on the side of the Federation anyway, due to their hatred of the Romulans. If that happened, then the Romulans lose badly.