It's never long enough. However, ever since I've seen those shots on Flickr from this lady with the 300mm+1.4, I can't stop thinking about it....Plus I'll have it on the D7100 which adds another 1.5X FOV to the equation.
The main question as far as i can see is will it be long enough for you,the 70-200 2.8 takes some beating on IQ but it wouldn't be long enough,i could have got the new 300mm pf but even with the converter and cropping it would still be too short in my situation,i end up doing 50% crops on 600 for small birds most of the time.
If 300 and 1.4 would have the reach for you than that could be your best bet,have a good look at small birds on your 300 and see how much your cropping that should tell you.