Upload & Sell: Off
Not as strange as it seems. Imagine to slug around two sets of TCs. one for Nikon, one for Sigma. That one reason alone convinces me to stay 100% Nikon.
Sorry, I have to agree with others who have questioned the logic..
I completely understand if you want to stay with Nikon glass as a matter of loyalty or if you have where-with-all financially, but I use the Sigma 500, along with a Sigma 300 and a bunch of other Sigma lenses. I own two Sigma TC's as well as a modified TC-14e specifically for the 500. The 500 & TC 14e work great together. Carrying one additional TC in one pocket and having $4000 extra cash in the other seems to be a far cry from your description of slugging around two sets of TC's..
As others describe, the performance of the 500 & TC14e is just fine on the D300, 700 and D3. Since I bird with two bodies, it's easier to have one tc mounted on the 500/D3 combo and a Sigma 1.4 in my TT pouch in case I want to throw it on the second body.
With the tab removal the tc14e now mounts on lenses that Nikon didn't intend them to mount on.. the 70-300VR, 80-200 etc.
Interestingly, I tried the tape mod on one of my Sigma TC's so it would AF on the 500, but it subsequently, it would chatter when mounted on other Sigma lenses which is what led me to keeping the Sigma TC's for the shorter Sigma lenses and a dedicated TC14e for the 500 and other Nikon lenses..