Your post interested me. I also have a 70-300 which wasn't quite doing it for me when shooting track events, since I need a little more wide angle in some shots. Not much fun trying to change lenses or even cameras in a split second when following a group from the start to the finish in a 100M Sprint, for example.
I recently purchased an 18-300mm and have just started trying to learn its pros & cons, and how best to use it. Most complaints I've heard is about soft focus at 300mm wide open.
Here is a shot off my deck at a hummingbird on my Rose of Sharon "tree". First at full shot, & then super cropped. I guess I need to do the same thing with my 70-300 & my 18-200mm. If anyone knows the ID of this hummer, I'd be interested. There are not supposed to be anything in this area except Ruby Throats.
BTW, This was shot handheld. 1/125 -- f9 -- Center WeightedAverage (Forgot to change that) -- 240mm (Tried not to shoot full focal length) -- Auto Exposure (YIKES! I grabbed the camera really fast when I saw the hummer & forgot to reset from when I had someone else shooting me at an indoor event with flash) Another test will be soon.