Many, but not all. And how could Scotty know how fast the enemy can move under cloak? He wouldn't have been able to observe the vessel under cloak, by definition - his powerplant type and capacity assessments would supposedly come from the moments the ship was briefly visible.
Scotty could also be basing his assessment on the so far observed movements of the enemy - that is, the brief observations of the vessel's presence at various locations separated by the invisible transit runs. But that would be awfully unhelpful and speculative of the engineer, whose contribution should come from understanding the engineering aspects of the opponent. Not to mention that said movements actually support the idea of at least moderate FTL performance.
If anything, the heroes at that point would appear to agree on the enemy ship being immensely powerful - capable of firing powerful weapons, capable of running the supposedly power-hungry cloak. While it's evident that only one of those two feats can be accomplished at a time, pairing warp drive with either of the superskills would appear to be easy enough - the Enterprise does not have such powerful weapons and her CSO is impressed by the energy requirements of the cloak, yet the Enterprise can maintain high warp, so high warp apparently does not call for such extreme power, and thus could be added to either plasma gun firing or cloaking.
Sure, there's nothing to say that the power balance indicated in the McMaster plans couldn't be true. But that's not something Scotty could divine at that point.
...which IMO calls into question the original line of dialog, as well. How would Scotty know that their power is only simple impulse and be so sure about it, at all?
I think it's reasonable that Spock and Scotty are basing their opinions on not only what they observe, but what their training and experience tell them. Scotty knows that the BoP became visible to fire on the outpost, and presumably they knew how fast it was going, too. He can estimate the total power that the BoP can generate, from the assumption that the lion's share of power output was used to fire the weapon. From knowing what sort of power is needed for invisibility, I don't think it's a stupid premise that he can see that their generators just can't do that and warp at the time, especially if all the other transient observations they have fall into line with predictions according to such an envelope.