Let the benchmarks below speak for themselves. There is no penalty using 4th slot with RDIMMS.
58GB - 16x16x16x8 @1333mhz
https://browser.geekbench.com/v4/cpu/8813349
48GB - 16x16x16 @ 1333mhz
https://browser.geekbench.com/v4/cpu/8807752
Apple never shipped with R-DIMMS. OWC doesn't sell R-DIMMS. R-DIMMS don't work in 99% of desktop PC's. Why? It was generally too expensive. Ebay is flooded with pulls from decommissioned servers and it's dirt cheap.
58GB - 16x16x16x8 @1333mhz
https://browser.geekbench.com/v4/cpu/8813349
48GB - 16x16x16 @ 1333mhz
https://browser.geekbench.com/v4/cpu/8807752
Apple never shipped with R-DIMMS. OWC doesn't sell R-DIMMS. R-DIMMS don't work in 99% of desktop PC's. Why? It was generally too expensive. Ebay is flooded with pulls from decommissioned servers and it's dirt cheap.