The problem with most of these budget zooms like the bigma or the host of other xxx-400mm zoom manufactured by sigma or tamron, they will severely let you down in mid to poor light conditions, especially when tracking aircraft in the Loop.
They are fine when the weather and light conditions are good, but they start to miss lots of shots when the light conditions start to fall off. Personally I would choose the canon 100-400 or nikon 80-400 over any of these lenses when it comes to the affordable end of the market, even if you are paying abit more.
hyakuhei recommended the sigma 120-300mm f2.8 which is a good lens, fast autofocus, heavy'sh (2.5kg's), but does have its problems, especially with build quality, personally better for motorsport than aircraft, but just about ticks the boxes in the affordable stacks.
Peter