In ME1:
Overkill (Level 1): Firing rifles generates 40% less heat and costs 20% less accuracy for 10 seconds.
Advanced Overkill (Level 8): Firing rifles generates 50% less heat and costs 30% less accuracy for 12 seconds.
Master Overkill (Level 12): Firing rifles generates 60% less heat and costs 40% less accuracy for 15 seconds.
In LE1:
Overkill (Level 1): Firing rifles generates 80% less heat and does 50% more damage for 6 seconds. Purges heat on use.
Advanced Overkill (Level 8): Firing rifles generates 90% less heat and does 75% more damage for 6 seconds. Purges heat on use.
Master Overkill (Level 12): Firing rifles generates no heat and does 100% more damage for 6 seconds. Purges heat on use.
The problem is that Master Overkill in LE1 is buggy and does not work.
If you play on a PC you can either use the "LE1 Community Patch" on Nexus mods or the "Master Overkill Fix (LE1)" mod, as well on Nexus, that is part of the community patch by now.
