Or "simple impulse" is able to keep up with a Warp 5 Enterprise?
This doesn't sound like a fruitful line of speculation. After all, it means that Scotty would have been dead wrong anyway: in claiming that their power was simple impulse, he was claiming that they couldn't
keep up with a warp 5 Enterprise
(and the other heroes agreed that yes, this would follow).
What does seem correctly evaluated in "Balance of Terror" is the performance of the Romulan ship, as the hero ship can
outrun her. And yes, this is severely at odds with "The Deadly Years".
The difference between the two episodes? The ships in "The Deadly Years" do not use cloaking devices (or, save for the first shot, those deadly plasma cloud weapons) in the fight! This probably keeps them safe from fuel consumption problems and free to use their powerful warp engines...