I’ll use my non-seasonal WW/Rend Barb - Meteorblade - as an example…
Here he is with five flawless royal diamonds in his chest / legs…
https://maxroll.gg/d3/d3planner/354099554
- Damage 14,331,407
- Toughness 609,987,835
- Recovery 37,466,704
Here he is with five marquise diamonds in his chest / legs…
https://maxroll.gg/d3/d3planner/293125120
- Damage 14,331,407
- Toughness 578,656,867
- Recovery 35,542,292
So, going from five marquise to five flawless royals gives him an increase of…
- Toughness - 609,987,835/578,656,867 = 1.0541
- Recovery - 37,466,704/35,542,292 = 1.0541
In other words, it’s 5.41% more mitigation having flawless royals rather than marquise.
So, yes, you really do gain something.