The "toy gun" drill is a good "eye-opener" drill to run, for people who have never tried to do anything like this before. Playboypenguin and The Tourist are right, in that the drill does not factor in the adrenaline rush and time delay associated with a real-life situation. BUT, the "toy gun" drill is a measure of the "best case scenario," and it will let you know whether or not you are quick enough, given enough alertness, preparedness, and aforethought of the threat, for the disarming technique to even be possible.
In other words, if you're running the "toy gun" drill to see if your martial arts skills "will protect you against a gun," forget it. You're wasting your time, because the drill is not sufficiently representative of the real-life situation. But, if you're trying to answer the question, "In a best case scenario, is it possible for me to get this guy before he can draw and shoot me?", then the "toy gun" drill is a pretty good test.