Well while I don't expect an ISD to stop on a dime, there's one bit that's very telling about the ISD's acceleration capabilities in RotJ.
The Imperial fleet is sent to the far side of Endor's moon, and ordered to wait. The Emperor knew that the rebel fleet was coming, but didn't have any sort of time scale for that. He simply knew they'd come. From this we can assume that the fleet was at something approaching a dead stop.
When the rebel fleet arrives, within moments the Imperial fleet swings around the moon and is in engagement range. While perhaps there was some dramatic editing involved, I can't bring myself to believe that the cut from "It's a trap" to the fleets engaging each other was a timelapse of several hours, so we know that the ISD (And, by extension, the Executor) can accelerate from nearly zero relative velocity to a very high speed within minutes, in order to swing around the moon so quickly.
Curtis Saxton did an excellent analysis over on his Star Wars Technical Commentaries site, and while his conclusions give some pretty massive capabilities of acceleration (In the order of thousands of Gs) which wouldn't work in a game environment, they still show that the ISD's speed and acceleration are nothing to be discounted.
Of course, being able to turn in an entirely different story
