Personally, I think this is a great challenge on many levels. First and foremost, I think it really assesses a Chef's skills. Not only are you testing their palate, but also their knowledge of cooking techniques and general skill. This is not simply a case where they can recreate a dish they've made hundreds of times before and know through trial and error that it should score big. (Hell, many of them can't even manage this) In this challenge, you have one real shot to follow in the footsteps of the best of the best.
I do think that the order of the challenges is a bit problematic though. You couldn't run this challenge at the start of the show because it would simply be a horrible zoo. There are too many hacks and wannabes at that point and I seriously doubt many of them could duplicate a cheeseburger they're only allowed to eat once. The problem with having the competition so late is that outside influences play a very large role in determining who went home. Since Stefan won the quickfire, he got to pick which dish to duplicate and he obviously picked something he was familiar with. I can only imagine the terror of being told to remake something that you have absolutely no fore-knowledge about. These dishes weren't the common everyday things that everyone would be expected to have experience with either. Past experience ended up playing a huge role, though it wasn't exactly insurmountable. Carla had never seen her dish before but managed to piece it together Sherlock Holmes like.
In the end, Jamie went home which was almost certainly a mistake as far as the competition as a whole goes. Lea is a talentless lump at this point and should have gotten the toss. If they were being judged upon their performance through the competition as a whole, it's clear that she would be gone. It's just that in this case, Jamie came up with something that was simply inedible. As someone who tried once to fry stripes of corned beef as steaks, I can assure you that it's simply impossible to eat anything that's like a salt lick. You just can't swallow it, it's so toxic.
If you noticed, the personality pivot also occurred either late last episode or in this one. All the 'jerks' have become less jerky. Everyone pulls together more. It happens almost every season and is just human nature. I'm not saying that Stefan has become lovable or anything, but I can't help liking the guy now. I even felt bad for Jamie and early on I thought she was a bitchasaurus.