Compatible EF Lenses w/ C70

[DPhotoR]

I am ready to buy this and have one on preorder. Thing is...I will be attaching a 300mm f/2.8 and 600mm f/4.0. Based on the information Canon released (below) I wonder what will happen when I attach one of those (via the 0.71x). Note in Statement II they ID compatible lenses, but then add that all EF lenses are compatible. What happens if you attach a "non optimum performance or non compatible" lens? If anyone can made sense of this, please advise.

Statement I:
The EF 24-70 2.8 II, 24-105 4.0 II, and 16-35 2.8 III are supported for lens correction with optimum performance." Not sure what that means for the remaining Canon EF lenses.

Statement II:
As of September 25, 2020, the following lenses are compatible: the EF16-35mm f/2.8L III USM (released in October 2016), the EF24-70mm f/2.8L II USM (released in September 2012), the EF24-105mm f/4L IS II USM (released in November 2016). Canon plans to gradually expand the list of compatible lenses by providing free-of-charge firmware updates. EF lenses are still compatible with the camera, but Canon recommends that firmware updates be installed in order to effectively utilize functions which link the camera and lens.

 

[DPhotoR]

I spoke to Canon and they explained that all Canon EF lenses will be compatible with the C70. However, not all of the functions, for example auto tracking, iris detection, etc may function on all lenses. Only the 3 aforementioned are have been verified to work with "optimum performance". Canon says they have to wait for the manual to be released.
In short, the 300mm and 600mm lenses will work with the C70, but they do not know what functions will or will not operate.