Our results are crystal clear: at least with the five brands we included in our roundup, the thermal compound has minimal impact in cooling performance. We tested only five different products (which really is a small sample), but the results we consistent.
From the five thermal compounds we tested, four of them shown practically the same performance. Only the Prolimatech compound behaved differently, but the difference was of only three celsius degrees, just one degree above our error margin.
Application of the thermal compound and proper air flow are more key factors... What would be important to determine / review, is the performance of a stock Intel or AMD thermal compound. This should show you should if there is truly an impact with aftermarket thermal compounds.
I have had good results using MX-4 and IC Diamond they both dropped my temps by 3-5C. AS5 is good compound too, but there are better and yes 2 to 5 degrees does not seem like much, but it is a difference and every degree counts sometimes when overclocking.
Today's processors put out far more heat .... looking at the last heat sink test done by BMR for example, the results ranged from 58 to 75 C which makes the impact more pronounced.
At what point does a difference in numbers become "significant", selling ya car, what's better .... getting a price of $4210 or $3755 .... how about $4000 .... or $4100 ?
Better number is better numbers.....would I fret after finding out that a new TM did 0.2C better than the one I just used in my build ? No, of course not, but next build, with them both costing $5, I'm going to buy the better product.