It's not the TIM that's the issue. The TIM has been tested and is actually quite good. The black adhesive that Intel uses to affix to IHS to the PCB is the issue. It causes the distance between the die and the IHS to be quite large and they need to use a lot of TIM to fill this gap. That reduces thermal conductivity. By delidding, you remove the glue entirely, and reduce the distance between the die and IHS, and by using a liquid metal TIM, you improve the thermal conductivity even more.