Not necessary, regarding the photography everything on the R5 Just Works™
This has been true of Canon cameras for years. Tony Northrup said the AF is now "better than Sony" which was probably the only lingering question for stills shooters moving to mirrorless. (And for him to say that you know it's good.) What other questions are there?
High ISO? Dominated by photon shot noise which is not subject to tech. There's about a 1ev difference between the worst and best FF sensors of the past decade, and the R5 will be near the top (best) of that. So comparable to a 5D4 or 1DX3, and about 0.5ev - 0.66ev better than the 5Ds/sR.
DR? At least as good as the 5D4, possibly 0.5-1ev better based on a Canon statement. Which would put it 1-2ev better than a 5Ds/sR.
Real world resolution/sharpness? The R5 samples are gorgeous and have the same phenomenally detailed/sharp look of the 5Ds and 5DsR samples from 5 years ago. AlanF will no doubt tease out any differences of the AA R5 45mp sensor versus the AA-less 5DsR 50mp sensor, but for 99% of people it's a moot point because the R5's IQ is simply excellent.
If you want to go high resolution 35mm and money is no object, buy an R5. If you want to go high resolution 35mm and money is an issue, go on eBay and pickup a new, gray market 5Ds for $1,100 (occasionally they drop to $1,000). With one the AF point magically follows your subject and you get live exposure preview. With the other you have to follow your subject and you get longer battery life. Otherwise they just work.