The Romulans were originally a minor power, behind the Federation's advances since being defeated and isolated after the Romulan war. In their first appearence ("Balance of Terror"), they had a ripped-off Starfleet ship with a bird painted on it and no warp drive. It was only their new cloaking device and plasma torpedo weapon that made them formiddable. Later they got better, Klingon-designed ships.
What is this about the ripped-off ship? Is this from the novels?
And no way did they have no warp drive. It might have been impusle power, but capable of generating warp fields, otherwise, they could never sustain a star empire.
From a bit cut from the "Balance of Terror" script (and ignored by later Treks):
INSERT - SHIP'S VIEWING SCREEN
The attacking vessel can now be seen definitely to be some modified version of a starship saucer main section... but with the dark markings on its underside which suggests a bird-of-prey with half-spread wings. And centered there is a threatening-looking "weapon tube" device. We see it in full size only momentarily... then it launches a torpedo-like bolt of blinding energy from the weapons tube.
You see it, Enterprise?! Starship design. Warn Earth...
... espionage, stolen our designs... traitors...
As for the warp drive, that's I think more of Trek's bad science. Insurrection had the line "Warp drive turned a bunch of Romulan thugs into an empire... a century ago..."