Results are quite clear; applying the right amount of thermal paste reduces temperature by up to 6.5°C. But one should not forget that the quality of the Artic Silver 5 is for sure higher than the thermal paste used by Apple, and this might also contribute to the lower temperatures measured after the modification.
In addition, Apple might have chosen the best ratio between: cost of thermal paste, amount to be added, easiness for manufacturing lines workers, and impact on MBPro lifespan.
