the true test for this lens is how well it will work together with TC
Considering it is f/6.3 at the long end, and the cameras which one would use this lens with are 45MP, I'd be happy to see the lens deliver details at the level of individual pixels in the image in practical real-world situations where the light is photogenic - without the use of a TC.
With a TC this lens would then be f/9 or f/13 wide open and it would add air-glass surfaces that potentially reflect some light. Flare could be an issue even without TC considering the lens already has 17 groups. f/9 is kind of imaginable to use in bright light but at least my shooting conditions are such that I have ended up selling my long f/5.6 lenses as the ISO has been too often at 12800-25600. With the longer focal length of 840 or 1200mm, one would more often want faster shutter speed as the subject is magnified and field of view is very narrow. Furthermore, the resulting small aperture doesn't make the job of autofocus easier.
I just don't see a case for TC use with this lens. For 600mm shots, f/6.3 is quite good in a portable lens. When an incredible offer is made, that is already seemingly too good to be true, what invariably happens is people start asking for more. ;-)