First, I'll say I have never used a D300. I have used a D7000 for two years, and since August, a D800. I shoot mostly wildlife also.
You said you don't like the slow shutter speed of the D800. Do you mean the frame rate? The camera will shoot 4 fps in FX mode, 5 fps in either 1.2 crop or 1.5 crop (DX).
I think there have only been a few times I wished for more fps. I figure if a bird moves fast enough that I can't capture a few good shots with 4 or 5 fps, chances are I'm not going to have time to lock focus on it anyways.
There is so much more to gain from the D800 that fps should not be the only factor to look at. I think the AF system is fantastic. I use AF-C 21 points for birds in flight and get great results. I use a AF S 300mm f4 with a 2X converter (f8 max aperture) and the AF system works great. My D7000 struggles to focus this setup.