So just picked up MCH and this heat gauge is a weird mechanic to me.

First off the description. It says when overheated it increases damage by 10%. Is this 10% on top of the 5% from gauss barrel? Or does it replace the 5% from gauss barrel, in other words, is overheated, in total, a 15% damage increase or a 10% damage increase?

Next, do I want to overheat it? Overheat seems to be a bonus and, at least right now at 60, there seem to be no advantages to not overheating it. Yet all my skills want to stop me overheating it. Ammo stops the heat increase, quick reload reduces the heat and I just got cooldown which greatly reduces the heat for an attack that's weaker than clean shot.