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This would be my response also. Get those mids and putters to fly for you then work up to Fairway and Distance Drivers. They'll show you what your doing wrong...
If you don't have the arm speed to throw a Katana or Vulcan... it's just gonna be over stable for you. You have to throw the disc at the speed it's made to fly, in order for it to do what it's supposed to.
I'll echo the stay away from the high speed discs advice.
When you practice, practice with putters and mids. Most players that are throwing 300+ with ease are the same players who are throwing putters at least 150-200 and mids 250-300. And it's all in the technique. If you have technique good enough to throw a putter 200+ feet, you have technique that will translate to throwing a driver 350+.
Once you have the technique, then figuring out which high-speed drivers are appropriate for you is that much easier. That said, I swear by the Valkyrie as a great distance disc for beginners and/or for (lacking a better term) "weenie" arms. If you can't throw a Valkyrie 300 feet, you've got no shot at doing it with a faster disc like a Katana or Vulcan.