What kind of dog is Diesel? Forgiveme, I remember seeing your introduction post, but I just woke up and my brain's a little sloshy, lol!
The reason why I ask, if he is a GREAT DANE, then it is not likely that you will ever get him slamming himself down at lightning speeds. Danes just aren't physically capable of doing things at lightning speeds. You can get them fast--for Danes--but not Collie fast. Not that they won't try. Their bodies just won't let them.
A lot of people don't like to work their dogs at their peak of excitement. I personally disagree--you just have to work carefully with your dog to develop control even when they are their craziest. That being said, I do have a Border Collie and a BC/ACD mix, so they are both crazy dogs anyway. But, they work their absolute best when they are completely wired. So if I'm doing a demo or something, I get them super riled up and excited. I'm working with 4 dogs of various energy levels now for a coworker, but 3 of them I do a lot of training at their highest energy level.
You could try a couple of things. To get super fast sits, I work on spins. Once your dog has fast spins and can do 3 or 4 in succession, then I will cue 3 or 4 spins one right after another, and then immediately ask for a sit. They are already in "speed mode" so the sit right after the spins will also be fast. I also reward fast sits greatly and slow sits get really boring treats.
So, perhaps you could try getting him a little more excited. Okay, just checked out your intro post because it was driving me crazy, lol...so Diesel is a 14 month old Dobie. ^^
You could also go back as if you were re-teaching it. Do all the same things you did before, but make sure that you're clicking the second his butt hits the ground. The faster his butt hits the ground, the faster he gets the treat, so he should speed himself up. If you click too early you will be slowing him down and/or weakening the meaning of the command. So just make sure you're clicking only when his butt hits the ground.
From there you'll just give really great rewards for fast downs, and really boring ones for slow downs.
Hope this helps. Good luck!