The Treaty of Algeron would be nulled and have no standing. Thus, whilst the Romulans will have a brief advantage due to the cloaking device, there is nothing to prevent the UFP from obtaining its own cloaking tech. And we saw in The Pegasus that this is superior to anything (that we know of) the Romulans held.
I've a problem with this statement. The Treaty of Algeron is mentioned several times, but its terms are never made plain (beyond the Federation's not being allowed to have cloaking technology). We don't know what concessions the Romulans were forced to make in order for the treaty to be signed.
Although the treaty may have prevented the Federation from developing cloaking technology, it's also entirely possible that it contained provisions preventing the Romulans from making significant improvements to their existing cloaks, as well as a developing other weapons that would have given them an advantage over the Federation.
Subspace weapons were banned by the Khitomer Accords: the Romulans were present at this conference. It's entirely within the realm of probability that a similar clause was included in the Treaty of Algeron. Granted, this is an entirely hypothetical scenario and has no basis in fact. But my point in mentioning this is that we cannot assume that breaking the treaty would benefit only the Federation. Treaties are based on compromise and mutual concessions: we don't know what the Romulans were forced to concede; therefore, we can't know how they'd benefit if the treaty were abandoned.