Switch from AFS 300/4 to Sigma 120-300mm f/2.8 APO EX HSM DG OS ?

I had the original 120-300 and it was a superb lens , I used it for motorsport, I think it would be a very good alternative to your 2 current lenses and you would probably have a lighter camera bag to boot. I use a monopod and ball head set loose to support the weight, The lens is also good with a 1.4x Sigma but not with the 2X.

Here is a shot taken of Alex Zanardi, Indycar and Olympic champion at Brands Hatch.

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."