Swarmkeeper
Hero
I wouldn't consider ending early as a punishment, particularly if I were at a convention. That just allows me to spend a little more time in the dealer room, getting food, or getting to my next game. It also allows the players more time after play to just talk about how things went as well as other topics without it interfering with the game. There's usually a fair amount of that at AL games I've been involved in.
If they had managed to skip a third of the module by smart play and I saw on my clock that I was running WAY ahead of time, I'd probably milk more out of the final encounter - more bombastic behavior from the BBEG, more joking about and otherwise having a good time. I might even let the players know about the reduced time pressure as well.
I mean, maybe punishment was too strong a word but if I signed up (and paid) to play in AL for 4 hours at the LFGS and the play time was cut short 1-2 hours because our tactics were solid... that's not ok, IMO.
I see your point about cons, though. If the group completes the adventure in short order there's plenty of other attractions and I wasn't paying just for that one game.