I had a similar experience with my 400/5.6, 1.4x and 5DsR. In tests I found that the teleconverter (mk1) didn't produce better files than simply cropping 1.4x. I regret selling the 1.4x as my R gets a lot more use than the 5DsR.
With my 5DsR I found I could use my 1.4X TC III with my 100-400mm II, but when used with my 400mm f/5.6 the CA was not acceptable. What I would like to know is how well does IBIS work on an R5 with the 400mm f/5.6. Does it stabilize the EVF well, and how many stops of stabilization does it actually provide to the digital image?
Thank you all above for your advice.
You shift me towards an early adoptation of the R5/6.
All three of you seem to point to - which was my underlying concern - a solution based solely on R-system being preferable compared to mixing EF and R-system.
Especially when the timing is as it is. (I guess I shold have invested in the 400mm DO much earlier - but that is another story).
Speculation for me is, that future big whites will be RF-based and are likely to be even better on R-cameras than current EF lenses.
So whether to go with the already announced 100-500 mm or with some yet to come bigger whites can be open options for now.
I have a Sigma EF 100-400 and it works like magic on the R6. I tested it side-by-side with the Canon 100-400 f/5.6 and it gave the $2k lens a serious run for its money. I suspect you could throw nearly any EF lens on either the R6 or R5 and they will work flawlessly. The cameras are just that good.