What Scotty says is that the Romulan power generation
is "simple impulse" which is not the same thing as their propulsion
being simple impulse. All of the available evidence suggests that building a sublight ship as your new weapon would be incredibly silly. The Enterprise had to go to warp to outrun just the plasma weapon's maximum range, so it doesn't seem likely that the BOP wouldn't have had either warp or some other FTL system. It simply wouldn't be practical.