The 10900K @ 5.1/4.8 doesn't see over 56c while gaming on any core
I will push 5.3/5.0 with direct-die, and probably 5.2/4.9 with the Copper IHS. Eventually you start pumping enough voltage and the cooling can't keep up. I'm at 1.26v under load for 5.1. I think 5.2 will be around 1.31, and 5.3 around 1.375-1.39, but I haven't tried 5.3 at all yet.
Lapping the cold plate on the cooler should make temps better regardless, though.