The performance improvement can actually be pretty significant, depending on the exact setup of course. In my case I delidded 3 Intel CPUs a few years back (i9-9900k and similar vintages) and replaced the thermal paste under the lid with liquid metal (from Thermal Grizzly) and also used liquid metal between the outside of the lid and either a water cooler block or a heatsink and fan. I forget the exact figures but I wanna say idle and even maximum load temps were around 20 degrees cooler... and all 3 were max overclocked to 5GHz. In fact I still use all three as network storage servers now.
