... and the performance, both AF and optical, with the x2 attached. And more fundamentally, if the 2x will physically fit which I would guess is unlikely.
The best route to that combination of focal length and aperture would be the 70-200/2.8 with a 1.4x, giving you a 98-280 f/4. Slightly more range and only a 1.4x so very likely better performance. Also has the advantage of not being a rumour! Again the problem might be compatibility - Bryan Carnathan notes
here: "the RF 70-200's rear lens element is quite shallow when the lens is retracted, leaving little space for an extender to be inserted into the back of the lens." He wrote this before the RF Extenders were announced, and we now know they have quite significant projections into the lens. Surely, you might say, Canon wouldn't take away the Extender compatibility we had with the EF 70-200s? Let's hope not, but I wouldn't bank on it :-(.