Peter, I see you have a 100-400L. For me the biggest weakness in the Nikon line-up is the lack of a high quality, affordable lens to compete with Canon's 100-400. Sure the 200-400 f/4VR II is excellent, but it's also $7000! Also, IMO the Nikon 80-400 f/4.5-5.6 isn't in the same league IQ wise as the Canon 100-400 and neither are the Sigma options.
If the Nikon 300mm f/4 with a 1.4X t-con produces good IQ it might be the best economical option to get to 400+mm. AF might be a bit slow though.
I'm back after a night's sleep! Yes, I haven't heard of anything comparable either. It wouldn't trouble me for a while, as I use the FF camera for wide angle stuff and keep the crop for telephoto. That's currently the 5D (with 12-24 and 24-105) and 7D (with 100-400). I'd stick to that with the D800E, using a new Sigma 12-24 and the Nikkor 24-120. But in due course it would probably become an irritant.
I'll tell you another thing about the Nikkor 80-400. Not only may it be inferior to the 100-400L in IQ, it's also inferior in weather sealing. I know of three 80-400s here that have succombed to internal mould, and every one of them was purchased more recently than my 100-400. One owner is so fed up with it (because Nikon won't help with repair or replacement by even one dollar) that he said he's switching to Canon - purely on the strength of my 100-400L!