I love my TC17 and have used it on my D7000 (equivalent pixel density) fairly extensively. I have especially relied on the TC17E since shooting more on FX these days. Likewise on my 70-200 I & II I always try and stop down to f6.3 at minimum. On my 500 though I have shot it wide-open pretty extensively too.
We'll see, I haven't shot a TC17 on a D800, but will keep everyone posted. I hope to be doing some shooting with that combo this weekend.