Upload & Sell: On
I had the 100-400L, and I loved the weight of it. However, once I got the 70-200 MKII and compared them, I just couldn't justify keeping it. For outside stuff where I use it, mainly at racetracks, it isn't uncommon to be stopping down a little anyways, so the IQ difference really starts to disappear IMO. Granted, I had an older 100-400L, I hear the newer ones are a little sharper.
As for focus tracking, the 70-200 MKII + 2xTCII can track a racecar at 180mph trackside without issue...the birders might have a different opinion on this, but for me it works fine.
In full disclosure, I do also have a 400mm 5.6L that I carry with me, and use for 400mm as well as 560mm with a 1.4TC...if I didn't have this lens, and did a lot of work at 400mm, maybe I'd have a different opinion on this.