I shoot pretty much what you do mate and have had the 800 for a while now, just got back from a week in the Hebrides.
My thoughts, the AF is better than the 700, it crucifies any bad glass and habits you may have. You have to get serious shutter speed if hand holding, it is very unforgiving in that regards. I was photographing a female hen harrier hand held in strong wind, no option available to me, the results would have been better with my 700. However, I got a curlew hunting and eating a crab, fairly large crop, the 700 would not have produced that result.
So, I keep both, they have good points and bad points. But by christ can you crop into the D800 shots, it makes the 500 more like a 1000.
FPS does not bother me that much any more, it is handy on occasions, but the 700 covers that.The high ISO quality is very close to the 700.
I got fed up of waiting for the mythical new crop body, the D800 covers that pretty well.You can shoot in DX mode if you want higher FPS and want smaller files, or the various other modes I have the Fn button set to alter that easily enough.
It is a good camera, you can come up and have a bash with mine if you want, before you buy.