It depends, do you really need the extra reach? what other lenses do you have? can you get the 2x TC III for a little more or not? do you need to hold it for long periods? can you get/do something better with the money?
I posted this in Critical View of 70-200 f/2.8 mkii+2xTC III its not a great test shot, but i intend to make more, i had a used 100-400 for a brief time (2 months = 1400 shots) and though i liked it, i had a chance to replace it along with some lenses with the 70-200 2.8 IS II + 2x TC II, tbh i like the combination, though as you know its a bit heavy.