Coming to the party late, but great choice.
I went long lens hunting last year.  Had an old 200-400 Tamron and wanted better.  Looked at the Tammy, Siggy and Nikon.  Bought the 200-500 after trying all three on my D610.  Now also have a D7100 and my 200-500 pretty much lives on the D7100.  
I also have a Kenko 300 1.4 TC and it works fine on either camera, but I can hear the focusing mechanism working in the D7100 - no similar sound out of the D610. Been told not to worry about it.  Both cameras manage well with the TC.  Yes you lose a stop of light.  And A.F. is slowed, in some situations - or so I'm told.  It does add a nice bit of reach to the 200-500 on both cameras.  But it isn't necessary.  With your mega pixels you can just crop-a-lot.  
I did find that for hiking I like my big lens on a monopod with a gimbal head.  Went for the Jobu Design BWG Micro Gimbal.  It does need a good ballhead as it is a ballhead adaptor not true gimbal but works very nicely on top of my monopod.  For quick walks and grab and shoots, hand held is fine.  And a good strap (attached to the lens not the camera) will make hiking nicer.  
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