Thought I would jump back into this thread. Shot at Silverstone overnight last weekend and it's incredible how much difference a single stop makes in incredibly tough conditions. I was panning at around 11pm at night, when I had my TC on (which makes my lens an f4) it was difficult to see in the viewfinder and you hardly got any ambient light in to at least get an outline of the cars. Took the TC off to give me f2.8, the difference was staggering.
I always knew why single stops at this end cost so much extra (particularly at 200mm +), but it was the first time I really saw the benefit. If my 50mm 1.8 could focus quickly it would have been awesome in those conditions.