I'm wondering if its worth the effort?
Welcome, Art !
Yes, it worth it, even more so you already have one.
I bought one already modified, as the difference between an original one is not significant. I have used it on my former D700 and now on my Df. I have pictures but I have to look for in my archives.
However, you will need good light when shooting moving subjects with slow lenses like your 400mm.
It works very well with my old 300mm F/4 ED (if the focus is already set up on the right range of DOF, as you have certainly noticed).
As for the TC, I made this very basic surgery with mine (old version TC-14 AFI). Very easy, but make sure you don't remove these two very thin metal rings just behind the mount as it is very tricky to put them back.
JIS crewdrivers advised.
I can post some pictures of the surgery on request if you need.
Enjoy !
Francis.
PS : we have an indisputable specialist here on NG : a great Dane from Denmark...(aka Dr Lens, aka Erik Lund)