A new lens for my kit, Canon 300mm f4L IS

Last winter as my shooting evolved to encompass a lot more backcountry shooting, I increasingly found myself shooting at long end of my 70-200mm F2.8 L IS.  Half way through the season i purchased a Canon 1.4x Teleconverter  to use with it , creating a 280mm F4.  Usable in terms of image quality but not great, I was left a little disappointed unless the lens was stopped down to f8 and that’s a pretty tall order when you are shooting action requiring at least 1/1000 shutter speed. This summer then i decided to pick up something that was naively a