The reason perhaps in "BOT" that the Romulan's can only go sub-light (which I do believe was the intent) was because it was a new design in which going faster than the speed of light would make them visible on sensors. Similar to the F-117 Stealth fighter. In this age when we have planes that move many times the speed of sound; the Stealth fighter was purposely designed to travel sub-sonic because if not it would be detected. Something similar could be (all conjecture of course) the reason why the Romulan's specifically design this "BOT" ship to travel slower the light speed because they believed it would keep them undetectable. Since the design obviously failed in "BOT" they went back to using the warp driven ships as we saw in "The Deadly Years." I just read the James Blish adaptation (which most of his adaptations were based off of original story's) and in it he mentions that the Romulan empire was just a solar system so sub-light speeds would not be a problem for the "BOT" mission.