I'm sure the good quality pivot joints don't start at XT. I just don't have other intermediary parts to test with. I've used an SLX derailleur and shifters in the past and the shifting consistency didn't feel worse in any way. Maybe it goes down to Deore. Maybe even to CUES. The CUES parts look really well made. But so does the Alivio so unless one can inspect it with their hands, it's not clear. :D
All that makes me curious about the quality of the newer "value" brands like Microshift, S-Road, Sensah, Ltwoo. If those have stiff joints, they may be a worthy upgrade for cheap setups.