Having just bought the current 100-400L recently, I considered the 70-200 f/2.8 IS II with extenders as an option. But honestly, while it would be way better from (70)100-200, and pretty darn good out to 280, ultimately everything I saw said that the 100-400L had it beat for IQ from 280-400. Plus you could then add a 1.4 extender (loosing AF) onto the 100-400L and get out to 560 mm, if needed. Then throw in the fact that AF slows way down with extenders, I bought the current 100-400L.
Finally, I'd expect the "new" model to perform even better than the current model. If they bring up the 100-400L to the current level (4 stop IS, weather sealing, IQ, etc), it is going to be one great lens.