Shall be ordering 400mm f2.8 (supply issues) or a 500mm havent yet decided in a couple of months (once i have had a chance to test the 2.8), However in the mean time can get hold of a Sigma 300mm f2.8 to use for weekend on test, but id be using it at 600mm, but will the Sigma work with Canon 2 times convertor? not talking just about fitment but mean AF metering etc? Anyone tried, or will i need to hire a 2x Sigma.