Wow, lots of great insight on the long lens topic. The explanation regarding 3" glass makes sense. Problem is, I like Ed's response about having the versatility of a zoom (100-400) for anything but birds, which I don't shoot.
So I guess with a 100-400 and a 2x TC, you'd get the range of 200-800 with 2 mm focal length increments. Seems good to me.
It's hard to follow you guys with the huge lenses. I think I will follow the "start small" and see what goes from there...
I got the Motorsport photography bug kicking in and want to shoot these events with my full frame 6D as the up close and personal detail should be awesome. Not as awesome as Dave's "biker" shot, but that lens is too much lens for me just getting into this.
I'm with Dave on the "more pixels on the subject" approach, as I usually dislike heavy cropping, being old school and all wanting to get my composition as I want it when I am shooting, not in post.
I initially inquired about the 400L, and now I am thinking a 100-400L might be the answer. I would still use my 24-105, the 40mm Pancake, or the TS-E 17 for the pits and detail shots.
I will go to another race in a few weekends, might rent the 100-400L and give it a whirl.