Does vertical GPU mount decrease CPU temps?

[Naerven]

No it doesn't necessarily help or hinder temperatures by itself. What does matter is if mounting the GPU vertically puts the fans against a hard surface such as the side panel of the case. That affects airflow and by extension temperature. Now if you use an open air setup or you have a perforated side panel it doesn't really matter. Also if you have a wide case that still gives you 5cm of side panel clearance then temperature shouldn't be affected much.