I'm gonna beg to differ on the pulling closer to the chest. I think that's why you're getting to 400' now. I see the problem as an early release. If you look at MB's video, he talks about "releasing here instead of *here*, and doing that adds about another 50-100' to your throw"...something like that.
What I see you doing is not starting your throw from your hips and legs. You seem to pull with your arm the whole way, pull close to your chest and get some good wrist snap, but when the disc comes out, you're not chest facing the target and the disc is gone while your arm is still on the left side of your body. Your acceleration is from the reachback. So... by starting the rotation with the hips and lower body and building upwards, you should be able to hold the arm pull until the disc is around your chest area, then accelerate. That should give you some more distance.
Honestly, I wouldn't put 500' out of reach, but I think that you'd need to be throwing well to do it. If you incorporate your hips and rotate with more power/speed, I think you'd get into the 450' range regularly. The other will just depend on that unpredictable factor of how well can you consistently repeat a specific action.
I'm also thinking there might be a tad bit of nose up as well, considering how fast those discs climb. Although it could be the landscape or just that it's hard to see them. I'm not talking about anything major, but those flat discs looked like they should have been 10'-15' off the ground and they were well above that.
I think that you get your weight forward more on the hyzer flip. On the other shots you're more apt to be upright, but that's part of the shot. If you look at the last throw, right before you release, you'll see that you're off your center axis (leaning backwards a little) as you pull through. Perhaps that's what JR was referencing.
Another thing you could improve is how you use your momentum. If you watch, you'll see you take your couple steps, your x-step, then pause and pull. The run up should be used to prime your hips. It's helping you by giving you some backwards/forwards rocking motion. You need to be twisting and moving from back to front. Think of yourself like a rubber band and you wind yourself up, then uncoil. It's he uncoiling that gives you the most power and keeping the disc close to your chest all the way through just harnesses it.
This is a great shot of GG to show how he harnesses everything. I'd even wager he can do better as well.http://www.youtube.com/watch?v=SJyq5xvBS0w#t=0m52s