I think you need to decide one way or the other if you're going to slide or plant. If you're intending to slide, but your shoe just won't slide, it will cause you to have little balance thus affecting your shot. However, if you throw the ball intending to plant your foot, you should be able to have more balance because you're prepared to plant. I would recommend going with more of a slide than plant because (from experience) planting can cause knee problems. Either way, moving up to a better shoe could only help you.
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