Spock mentions that the Earth-Romulan war was fought with "primitive nuclear weapons" in the opener of "Balance of Terror" and later Scotty says the Romulan ship is "powered by pure impulse." Most fans considered this to mean the Romulans didn't have warp drive until their later dealings with the Klingons (whatever that was). That assumption, when you stop to think about it, is pretty silly when you figure that 1) The Romulans were an interstellar power by at least the 22nd century and 2) They fought a fairly extensive interstellar war with Earth.
i think you forget that watching TOS even once i seem to remember that there was no UFP until later into season 1 and start of season 2, spock smiling at the beginning, the vulcanians?, warp 10 being exceeded easily by the enterprise.... and so on.
it is true that romulans at that time and earlier were using impulse ships to travel, in exchange to use their massive plasma weapon, other variants of the bird of prey may not of had this weapon and can travel at warp as it was never touched on in canon, stuff like this is where the problem begins.