Look to me that your pulling back from above your shoulder loosing all your speed trying to get it flat on release. Try increasing your speed from between your shoulders, like you pull starting a motor. Keep your hand below your shoulder the whole time. Also you might try a pro katana, I was able to boost my distance from 300-340 just with that disc. now im maxing out at about 350-370 on avg.
I should have mentioned this in the details at YT. I'm using the power grip with my thumb positioned just off the rim and on the top plate of the disc, as suggested by Will Schusterick (he, Dave Feldberg, Sheila Kirkham and Ross Brandt gave a clinic before the Memorial).
Discs with wide a "wingspan" - Boss, Katana, Nuke, etc., etc., etc. - don't feel comfortable to me and hyzer out on me.
REALLY reach back. When I finally saw someone do it, I could not believe the power that came out of it. For large bombs I actually turn my body and face the opposite direction of the basket, and my arm is STRAIGHT out. Practice walking an X step, and right before you throw, you should be facing backwards. I maxed out around 330' and after adjusting that, and a summer of practice I can get 430' on a good throw.
Marty Peters also has a nice vid from Discraft on YouTube about gaining distance. REMEMBER, you can only learn so much from a video, but the ideas can only help. One thing that I did pick up, though, was to turn your hips more, showing your target your hind end, then driving through. Here's the link directly to the part about this.
the two biggest thing i see is to first, keep your back striaght and second turn your hips. one thing i was told that helped me was that your arm can only follow what your hips do. by rotating your hips more you allow your self to reach back further which gives you arm more time to gain speed. keeping you back striaght will help you to reach back, keep your discs in the air longer, and will help keep you consistant.
It doesnt look like your planting your foot hard enough at the end of your x-step. try slowing down your run up until you can really put your weight into that leg and still have good balance. your body just seems to keep going in front of you instead of transferring that energy.
Don't try to learn max D with 13 speed drivers, even if it is the Katana or Vulcan. Learn to throw a 5 or 6 speed discs over 300'. Better yet, learn to throw a putter consistently to about 250' and learn to throw a Comet very well. Doing this will help you gain distance and teach you better form.