The biggest drawback with the 300mm F4 is it doesn't have VR so, using with the 1.4 or 2x converters requires a tripod. I suggest the Sigma 150-500mm F5-6.3 OS at about £750 might be your best bet. It weighs under 2kg and has a nice balance when attached to a camera. Like all lenses at the F6.3 it requires good light but will give you some stunning shots. I haven't got this lens but do have the Sigma 50-500mm F4-6.3 but without the OS. The advantage of the 50-500mm is it will take a converter but manual focus only and I've used mine with the Sigma 1.4 converter giving me a 700mm at F8.
