Chasing after more reach is a fools game. You will never have enough. The kit you have now is about as good as it gets optically so I assume you want better pics. You are not going to get great pics by standing off. You need to be in closer. Getting better with you fieldcraft is free. Better to spend your pennies on items that allow you to get within 40m of your target. Camo netting, gillie suit, or a pop-up blind.
Whilst I agree that getting closer to the subject where possible is the best solution, it's not always possible because of both physical obstacles and the fear of disturbance.
More reach will to a certain extent overcome some of those problems, however, the financial cost is there for all to see but if you can afford it, get a 600mm f4.
I wouldn't sell your 300mm though because reach alone is not the answer. A big lens is far less portable, has to be tripod mounted or at least rested on something and the field of vision is considerably reduced so finding the subject quickly is much more difficult. Flight shots are very difficult unless its slow moving or about to land on a perch.
There is also the added expense of new tripod, tripod head, bag, lens foot etc to think about too.
In my opinion extra MP's are not the answer either. The more you crop the less detail you have. The D800 looses much of it's advantage over the D300s because of it's FX body.
So, if you can afford a 600mm buy one but don't expect miracles. At the end of the day, finding good locations to shoot from might provide just as much satisfaction and save you a fortune.
Patience is a virtue as they say, the more time you put in the greater the chances and sooner or later that lucky break might bring that action shot right in front of you but, heaven forbid,you might even find it's too close to focus on with a big lens.
Good luck in your decision,Dave