...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.
Agreed - and by the same token, Kirk due to his experience probably knows fully well what weight to put on Scotty's brash "No question about it!" claims... They have good odds of hitting close to the mark, and that suffices.
But if everybody tacitly understands that Scotty speaks about the limitations of the enemy when invisible, why would the discussion fail to cover whether the enemy will stay
invisible? If the speed limitation is dependent on visibility, the key tactical question now would be "How to keep
them invisible, and still manage to hit them?"... Stiles' comments may derail the discussion for a moment, but we soon see the arguments concluded without the issue of a visible but fast enemy addressed at all.