Thu 15-Nov-12 12:09 AM | edited Thu 15-Nov-12 12:14 AM by MotoMannequin
In DX crop mode, the D800 will give basically similar performance to your D300 in low light/high ISO. Possibly slight advantage goes to D800 but I wouldn't expect more than 1 stop better performance. Shutter speed shouldn't be different but frame rate will be. The pixel count doesn't matter; in fact it's a red herring.
I think if you get a grip which allows 6fps in DX mode, you should be in a similar performance range. If you can get a 1.5x longer lens and shoot in FX mode then you should get better low light performance but you'll be limited to 4fps. For birders, longer lens generally isn't a realistic option.
Ultimately, if you're focal length limited and cropping deeper than DX already, then there really is still nothing that beats a D300/s right now, which is why I still use my D300s for birding, despite owning D7000 and D800. D5200 may prove to be an interesting package, but I still believe we won't see any camera thoroughly beat the D300 at this game, until there's a D400.