Don't think there are real world reviews or tests yet, I'm watching for them myself and expect them in the next week as a few shipping notices have started showing up.
Personally I wouldn't worry, I think it will probably be a solid lens.
It would be extremely strange if it is not equal or better than the EF 100-400 mk2 through most of the range and have usable 500 mm. Canon just doesn't really do too many L lenses that have part of the zoom range with junk quality and I wouldn't think this would be the case here. The RF lenses have been notably great performers in sharpness and stabilization. Hoping and expecting that to be true here too.