I am only just back from my vacation trip to Tokyo, Japan and already get this very nice email in my inbox. An enthusiasts going by the name "瀬文茶" has tested the temperature difference when delidding and replacing the TIM of a Haswell CP. The process is similar to what Der8auer has done with Ivy Bridge a couple months ago (forum thread). The results are rather impressive. With a Core i7 4770K clocked to 4.4GHz with 1.3V, the temperature under load goes from 90°C to 73°C when swapping out the stock TIM with Cool Laboratory Liquid Pro (82.0W/mk). That's a difference of 17°C! With the Prolimatech PK-3 (11.2W/mk), the result is only 7°C better, which is still quite good of course.
The process of removing the heatspreader does not really seem that complicated. Watch out for the small components close to the die, and perhaps add a little sticker over them to ensure they won't break off.