Playing and improvining fast 8th notes.

[Bin Hire]

The bassist on this clip (the second one)
http://www.youtube.com/watch?v=j9xf-...ext=1&index=24
is running 8th notes at roughly 336bpm when he solos.
I've watched it quite a few times and can get my right hand up to speed for a short time. No doubt practice will make my playing stronger and the time longer. I have practised the same three finger technique.
Here is my question.
What would members of this forum suggest to get my left hand playing good notes?
I am working with a metronome and varying the speeds from slow to faster.
Thank you.