but you're right, im sure starfleet went and spent tons of resources and time to develop an upgrade to a ship class that was outmatched in every category by the Sovereign.
and they said "F the treaty" and built them with cloaking devices...
Just because the Gal-X isn't balanced properly in-game doesn't mean it's functionally inferior to the Sovereign by design.
Anyway, even if there is a functioning Romulan government that's still named under the Treaty of Algeron, we've also been at war with that government as demonstrated by the Romulan front missions. When you're at open war with another government, treaties mean very little, especially regarding such valuable wartime technologies as a cloaking device.
As far as Warship Voyager goes... there's absolutely no way that the Federation is even aware of such a design. That information existed hundreds of years in the future, in a civilization that obviously had limited to no contact with Starfleet proper, else the records would already have been amended. It's even less plausible than having the Dauntless show up in-game, since at least Voyager had some information on that ship.