When practicing faster temps (250-350 range for me at this point) I have noticed it is much easier to play things that I have already worked out ahead of time. Since the unknown music isn't tensing me, I find it easy to concentrate on breathing, good posture and relaxing. This week instead of practicing unknown tunes at super fast tempos, I have simply been working on walking patterns in all 12 keys. Also my teacher was telling me yesterday at my lesson that when giant steps first started getting called on him, he played the exact same bassline on every chorus for the entire tune until he could work something out. He also showed me how he simplifies his basslines when the tempos get up there to minimize left hand movement. Despite being "simple" everything sounded great!