The 8000 is marketed as the combination of the best features from the 5000 and 9000 series pedals rolled into one. I wouldn't say it is better than the 9000, just different. The 8000 has a fixed rotor where as the 9000 has a floating rotor.
if you need something more heavy duty than a 9000 then you are not treating your equipment well. it probably isnt too much of a difference if there is any at all.