I always took Mr. Scott's line, " No question. Their power is simple impulse." is referring to the kind of power they have. They don't have Ion power, which seemed to wow everyone so much, they don't have Atomic power, which would be considered primitive at the time, it was a regular, normal type of power system that was powering their very irregular and new cloak and plasma torpedoes.
Isn't impulse power just a reflection of power generation levels? You can have an anti-matter engine without a warp engine after all. At most , they're using plasma to power internal systems and travel at sub light speeds?