The 10-17mm is (to put it bluntly) not a great lens. When you put it onto a 36MP camera, these flaws really start to show. I have used it with the D800 (once) an dwell not do so again!
You could shoot it in DX mode, but viewfinder framing is not great, and you are reduced to using a 36MP camera at 16MP! You would be better off with a D7200...
The Sigma 15mm is a great lens and can be used with a TC if you wish. Why do you want to use the 10-17mm?
I would be wary about the 8-15mm with the converter on the A7II until we find out more about its AF. It may be fine, it may be not so good....
Hmm.. yes, I can understand the 10-17mm not being great optically. And I'd prefer not to have to shoot in DX mode on the D810 because of the viewfinder framing limitations (which are absent with the A7RII's evf).
I was looking at a fisheye zoom vs just a Sigma 15 + TC because for the work I do, I need the versatility to shoot split shots, people portraits (individuals and groups), critters and reefscapes on the same dive. A 180 degree fisheye is a too wide for some of these shots from what I've discovered. The Sigma + TC might be too narrow for split shots and reefscapes -- I'm not sure, I haven't had a 124* fisheye before to really judge. I figured the 8-15 with a teleconverter would give me the flexibility since I can shoot anywhere between 180* and 124* in full frame mode and 156* to 80* in APS-C crop mode.