SPOCK: The Romulans were caught off guard, Captain. They're falling behind.
From this dialogue, it appears that the reason why they are falling behind is that they were caught off guard - thus, not
that their ships are slower.
It's easy to see why the Romulans would fall behind because of being surprised: they are giving up the chase already, because they know they have lost it by not starting in time. In mere seconds, the heroes will get out of the RNZ.
In contrast, it's difficult to see what significance "being caught off guard" would have if the Romulans are incapable of giving chase in all conditions. Indeed, it's difficult to see why the corbomite bluff was necessary in the first place, as Kirk immediately
calls for the escape maneuver, not after a breather that would have allowed the ship to regain high speed capabilities. Why not simply jump to warp eight and be done with it, if speed is key to escape?
Considering that the Commodore never ordered the Enterprise to accelerate to Warp 8 to escape the attack, we'll never know if the Romulans could've maintained position with the Enterprise.
This is true. All I am saying is that the episode cannot be used as evidence
that the Romulans were only able to match mediocre warp performance, and that at best it is consistent with that idea...
...And further that it isn't really all that likely that it would be consistent. After all, intercepting a target moving at a steady speed X calls for significant performance, not limited to being able to achieve X. One definitely needs high acceleration, which would make the Romulans a deadly threat to Kirk in "Balance of Terror" already and weaken the idea that Kirk can outrun them in combat. And, unless one has a really intricate deployment pattern for entrapment, and the ability to drop the target from speed X to much slower speeds with one shot en passant
, one really needs performance exceeding X to catch up with a ship at X - i.e. one needs to give chase at considerable speed margin vs. the speed of the target. Especially when it's apparent that steering out of the RNZ and to safety is a matter of minutes only even at warp five (Stocker was only minutes into the Zone before getting intercepted, if we're very generous about the cuts, and mere seconds if we aren't).
We have never quite seen any conventional opponent, least of all Romulans, force a Starfleet ship out of warp with one shot... Did Stocker perhaps panic and order Sulu to voluntarily reduce speed or stop dead? That would be a pretty odd reaction even from him, and something Sulu might well dare object to.