A FF 400 mm 5.6 with a 2x extender would be a f11 800 mm lens and due to weight and size only usable on a heavy tripod.
Actually I do own that combo from Leica. (400 5.6 Telyt lens head on Televit R rapid focus mount) Haven't used it for years due to weight and size. With the 2x Apoextender it could only be used at 1/2000th and faster, which was extremely limiting @ f11. On a bean bag maybe down to 1/500th, with luck. Actually the Telyt 400/6.3 and Noflexar-T 400-5.6 (another heavy beast) were a lot better. Forget about tripods on Safari.
As it is, MFT is the only system that has developed IS to the extent that makes it possible to shoot long lenses handheld at slow shutterspeeds, which makes quite a difference to wildlife photography. It is the GX8 +DG VarioElmar that offers 5 axis 5 stop IS. Do the maths
Actually, I met quite a few photographers on the way who used Canon the 100-400 L (which has a modest IS) and as soon as the light was not optimal they had to stop shooting whilst I was still pulling in good shots.
I used the Vario-Elmar 80-200 R + 2xAPOextender a couple of years ago, which gives good results, and is quite comparable to the APO 180 R 3.4 + extender,( which I owned before that) with the added advantage of zoom, but I was unimpressed by the handling, I preferred the 105-280 with 1.4x extender. However, both combos lack the capability of "getting the shot" that I find in the Panasonic combo. And I would not be up to 800 mm...
In general, though, a shallow DOF is not a desirable thing in wildlife photography. You need to get the eye in focus, obviously, but you lose sharpness in other essential components of your shot, be it the beak of a bird, the horn of an Antelope or the nose of a predator. Wide apertures are needed for exposure on long lenses and only rarely for thin DOF.
A FF 800-5.6 would have had the near ear of this Lion out of focus.