It's 25 potency bonus (which is a light buff from Heavensward where it was 20). + knowing it provides the next of a combo and that for the 2 phase of combo it's a +40, an ammunition can usually add a total of 25, 65 or 105 potency (according to was skill you used with).
About the Heat reduction removed, it's to make the heat build faster, as more heat means more Cooldown to use, or more Overheat to build. And before 62, you don't need to absolutely manage it with the current system.