My testing was with just one lens, the EF 600/4L IS II. I was going to write that I've offloaded all my EF lenses that take a TC so that is my only option, but upon reflection while that would be technically true since TS-E lenses aren't EF, I could actually repeat the test of EF vs RF extenders alone and combined with my TS-E 24L II, a wickedly sharp lens.

Could be that Bryan got a suboptimal copy of the 600 III.

Will be interesting to see what you learn!