Went from MX4 to PK1 and my temps have stayed exactly the same. Not a bad thing.
My temps were fine for my oc to begin with. It's okay for me as long as temps didn't rise after the switch.
After trying quite a few different greases in my years, the most important factor to me in a thermal grease is the workability, then the performance.
Most of these high performance greases I've tried all seem like they perform with 1c-2c difference at most between them.
At least from my own personal tests on the rigs I've built.
I've given up on the quest for the highest performing tim.
Spread-ability, staining, conductivity, cure time, difficultly cleaning off, and cpu lid or heatsink damage are all the most important factors in a tim anymore to me.
My personal favorite was MX2. Seemed like it had a good performance and great workability.
Pro PK1 is my new personal favorite. Spreads like a dream, cleans easy, no stains, no cure (barely), and performs the same to me as MX2 ,3 and 4.