If you are attracted to the 300 2.8ii and plan to use it with converters most of the time, the 400 2.8 is to heavy and to expensive for you, then maybe you should try the 500 4.0. This one is not so much more expensive and will have better AF and better IQ at 500 than the 300 at 420 and 600. It will be a better 700mm with extender than the 300mm with 2x and a compromised 1000mm with no or poor AF
My two main uses for this lens will be sideline football and birds/wildlife. From experience with others I know, a 500 is too long for football. I can believe this as I found that I rarely used my prior 100-400 at 400 on the sidelines. Also, that extra stop of light is critical to me for night games. So, a 500 would limit its use to just birds/wildlife for now. I'm thinking about buying the 300 and a couple years down the road adding a 600. Extenders will give me reach on the 5d, and I an gain even more reach with my 7d II for birds. I haven't shot anything yet with the 300 and 7d II.