I happened to see this episode again over the weekend. My impression of the "simple impulse" remark was not that it was related to their speed, but their power generation. The Romulans were constantly having to decide whether to run or to fight because they didn't have enough fuel to do all that they wanted. The Enterprise usually has no such fuel limitation.
KIRK: Yes, well gentlemen, the question still remains. Can we engage them with a reasonable possibility of victory?
SCOTT: No question. Their power is simple impulse.
KIRK: Meaning we can outrun them?
STILES: To be used in chasing them or retreating, sir?
Even in context, I think it works better relating to power generation. If your ship is faster, sure you could run away, but that doesn't mean you'd win in a fight. I think the ships are pretty evenly matched, but if the Romulans have a less efficient power generator, they'd run out of fuel first and be dead in the water.
I also found this line interesting:
DECIUS: We are beaten. Can it be true? The Praetor's finest and proudest flagship beaten.
It's their flagship? Interesting what that could possibly mean.