OKee.. Here's the scoop. I have the 70-200 2.8L IS USM. But it just doesn't have the reach I want. I put a 2X on it and it gets me closer but still is short at 400mm.
Now if I get a 300mm 2.8 It will get me to 600 with the 2X but will it make me happy or not?
Since I can get to 400mm with the Zoom-2X combo, would I just be better off with the 600mm that can get me to 1200mm if I ever want to?
The question you should be asking is 600 f4 vs 800 f5.6.
What you get will probably depend on how much light you get where you live; I'm in the UK so compromising on focal length to get that f4 aperture is probably worth it... but 600 is still pretty short if you're a birder.
You mentioned that 300mm is perfect for the sports cars. That may be but who needs f2.8 when you're panning? For freezing-the-action head-on shots f4 and 600 (or f5.6 and 800) will still give you sufficient shutter speed on a bright day and the steeper drop off you get from in-focus to out of focus areas you get from the longer focal lengths will help the cars 'pop'.
Don't listen to me though; my longest lens is a 300 f4.