Yes, the Canon EF 400mm f/2.8L is an amazingly fast and sharp lens.
What is the generally-agreed sweet spot on the lens? If it is sharpest at f/2.8, it has to be up there with the very best in angular resolution, making it extremely tele-convertable, about as sharp with a 2x as the 400/4DO II with a 1.4x, but at 2x the pixels-on-subject.
Much of the perceived "problems with TCs" of the past have a lot to do with AF, and AF just isn't a challenge like it used to be. The main issue is how the reporting of the TC affects maximum drive speed, and that's where various TCs can get a bit quirky.
For example, I can make the R5 focus with the Sigma 100-400/5-6.3 extended to 1120/18, and how I configure the TCs and/or tape them affect the max drive speed. Too fast, and there is occasional overshoot; too little and big changes in focus distance take a while to change (but focus is perfect). If we could define the AF drive speeds ourselves with a particular setup, we could probably optimize to our needs. As it stands, the TC attached to the lens is the only factor, and it and the lens together tell the camera, through the lens, what the max drive speed should be (and the body will factor EV level into that, too).