Upload & Sell: On
Jarmo, the last time I shot with 300 f/4 IS was 11 years ago, but I do remember my lens was not sharp with the 1.4xTC II at longer range. Then I replaced the IS version with the older non-IS model, and although that lens was sharper, it still didn't impress me with the TC on.
Therefore, my advice is to shoot 300 f/4 IS bare, and work on the ways to get closer to your targets. That is pretty much a universal principle (with some exceptions such as when shooting gators, bears, etc..) .