I have similar problems, but I have never been able to throw over 315 feet, usual drives are around 250-275 feet on the field. On the course, everything is usually bit shorter. I almost always throw stand still, but as I'm having trouble of getting the weight shift forward, I have been playing with an idea of doing a very slow x-step to get better rhythm to the throw. As I have major lack of consistency, it's hard to see the possible development. I'm currently going to limit the number of discs I throw at practice, so it's easier to see which part of the differences between a throw is from technique and which from change of disc. There is also so many issues that I know are wrong in my throw, that it's hard to try to correct them all at once. And to know where to start is difficult. For example it would be wasteful to practice heel pivot when I currently can't properly pivot at all...
I would actually be completely satisfied (at least for a while...) if I could just get my consistency up, if I could throw accurately to 80 meters or at least 60 meters with straight, hyzer and anhyser, that would be enough for most practical needs in the courses I play (no more double, triple or quadruple bogeys...) and it would make my game much more relaxed.
It is sometimes quite depressing to hear that some people get over the 300' or even 350' plateua naturally without much effort. I have been playing 3 years now, and reading and practicing quite much... I consider my self above average athletically, but I have recently figured out two problems in my play, which have been visible earlier in my other sport activities also. One is that I tend to go more or less 100%, like as a children, I always sprained my groin in the spring after snow melt, kicking the soccer ball with 100% power after 7 months break.
Another is that have problem of focusing on the target at the hit, both in soccer and in disc golf, I tend aim based on some spatial awareness instead of really looking at the target when kicking the ball or when throwing the disc. I look at the target before reach back, but I can't get the focus back on the target before the disc has already left the hand. If I try to force it, I get grip locked.
/end moaning and whining