70-200 f4L For Motorsport?

Another question (they just keep on coming!), How does a 1.4 extender work with this lens. I know that it will increase to 280mm and f/5.6, but is image quality and lens speed etc affected much/what are the cons of adding the extender to this lens? Also is there a big difference between the mkI and the mkIII 1.4x extenders? I am assuming the 1.4x mkIII will fit my canon 450d? Sorry for the jumble of questions!!!

Typically a 1.4x will slow focus, reduce contrast and sharpness, but not by much. Pretty sure there'll be some decent photos on here with the 70-200 + 1.4x. On my 70-200 and 300, the 1.4x is almost invisible.

Between the MI and the MIII extenders there is a big difference.
Between the MII and the MIII there isn't that much difference, but it still there. If you on a budget, the MII will do you well.

The 1.4 MII and III will not reduce IQ and focus speed very noticeably. Its almost a standard combo for 70-200 or 300 and above primes.

I have the MII and it works quite well. The focus slow down is not very different all, almost unnoticeable.