Sigma 500 4.5 and 1.4tc's with Nikon
You can read my thread but i have some news....
After the kenko experience i m now using the lens without any tc.
It is simply wonderful. ..so fast, so quick and with so many af points...and so low iso...Great prime!!
I prefer now to shot without tc and cropping later.
I will try soon the canon tcIII after the fw update.
My suggestion is...with crop cameras like 7100 forget the tc.
With ff cameras you should consider a very good tc so in your case nikon.
Think about it