I'm still around, just been a bit below par of late.
I have the Tamron SP 1.4 T/C and auto focus was almost impossible on my Tamron 70-200 f2.8, maybe locking in 1 in 10 attempts. I got a Sigma T/C with my 150mm Macro lens and decided to try it on the Tamron. It's called the 'Sigma APO Tele Converter 1.4x EX DG' and other than a little movement in the mount it works just fine.
Unfortunately the Tammy 70-200 had to go to provide funds for the 150-600 Sigma and I can tell you I miss it. I've shot with the current Nikon equivalent and IQ wise I don't see a lot in it, but the Nikon has an edge in focus acquisition.
If I was looking for some more reach for birds I'd be looking for a pre-loved Nikon 300mm f4. I had the pre VR model and it is as sharp as a tack. I swore I'd never sell it but sometimes we have to part with loved bits to buy more loved bits.
PS:
Both your cameras deserve quality lens.