300 f2.8 IS L II plus 2x Mk.III better than 100-400 f4.5/5.6 IS L?

I don't think Birdbrain was trying to do that, he was just using a familiar lens as a reference point.

I'm looking forward to the discussions that will follow when the 200-400/4 is actually available at last, so for the first time* we have a zoom lens of comparable quality and optical size to the 300/2.8LII with Extenders. They will perform pretty much the same function for anyone who primarily wants a long lens, with various trade-offs of convenience, versatility, weight and price.

(*@Garry - I know the Sigma 120-300/2.8 is not a million miles away and I seriously considered it, changing my mind 100 times before finally buying the Canon.)