JABW,
I would say forget about the 400 DO MkI - from what I understand it had glare issues because of the forward placement of the DO element. The superb MkII 400 DO however is another matter entirely - the DO element is upgraded and positioned further back in the lens solving any glare issues. It also plays very nicely with the 1.4xTC. About the only drawback to this lens is the comparitively large mfd.
If money were no object, and you want the ultimate in lightweight quality that is 'relatively' affordable, this is the way I would go. Even 2nd hand (if you can get one) would be nice - with the new, better, lighter 500 f4 MkIII coming on line, perhaps a few 400 DO II owners may trade up and be prepared to wear the extra half kilo or so (and likely near double the price for what they would unload their 400 DO II for).
As far as the 100-400 f5.6 IS II goes, you would again have to attach a 1.4x TC to get back near the reach of the Tammy. I will leave it to you to research the relative merits of the Canon zoom + TC versus the bare Tammy G2. Have a look at this review (only in comparison to the G1 though)
https://www.the-digital-picture.com/Reviews/Canon-EF-100-400mm-f-4.5-5.6-L-IS-II-USM-Lens.aspx
Interesting that the Canon holds onto f5 for a longer focal length, while the Tammy holds onto f5.6 for a longer focal length. One thing to note is that the Canon 100-400 II with 1.4x TC will be f8, whereas the Tammy will be f6.3 at 600mm - the extra 1/2 stop of light sure would be of benefit on the 7D's (I, II, or III). The Tammy G2's MTF chart is actually pretty good for what it is, even though it doesn't have the 'L' glass of the Canon.
The Tammy G1 especially, but even G2 sharpens up a bit more when the focal length is dropped back a touch, and the aperture stopped down to f8. I think it would be an entirely reasonable comparison to pit the Canon 100-400 + 1.4xTC (for 560mm @f8) against the Tammy G2 backed off to 560mm and stopped down to f8.
Physically, the Tammy is a bit longer, and only about 200grams heavier once the 1.4xTC for the Canon is taken into account, but it's MFD is over twice as long. I suppose the real question is how much time will be spent birding (especially the little geewhizzits where reach and speed is king) , and how much time will be spent at 100 or 150mm? (I think I've only ever taken about half a dozen shots at 150mm compared to ~10,000 odd at the long end
...
There's also always the Canon 200-600 f5.6 rumoured (for a long time!) to be on it's way too, just to complicate matters!

- though it won't be an 'L' glass category lens. It will be really interesting to see what sort of pricing and IQ this is at ....... :eat:
I can say that the Tammy G2 is a worthwhile upgrade over the G1 due to the improved IQ, shorter MFD, improved 3 mode VC, 3 ranges of AF distance limits, compatability with the dedicated Tammy 1.4xTC, and the 'tunability' to your particular camera body via the USB dock.
https://www.birdforum.net/showthread.php?t=330742
Arguably the Canon 100-400 f5.6 IS II is the better IQ lens, and would be more versatile (if you don't mind swapping TC's in and out. Also at its higher price point than the Tammy you could quite reasonably expect less unit to unit variation. Things like rig balance, handling and operation, AF speed, and portabilty are probably every bit as important as any optical quality differences.
I would say get the lens first, and then wait for the release of the 7D MkIII for which we may hear more about next year.
https://www.birdforum.net/showthread.php?p=3777548#post3777548
Chosun :gh:
P.s. ...... are you really wedded to the Canon system? Not a zoom, but a very nice lightweight rig could be had with the Nikon D500 and the just released 500mm f5.6 PF. I myself will be examining upgrades when the D500S and 600mm f5.6 PF become available :t: