Some quick calcs for field time to get max Heat attempt #2 because last time had a mistake:
Energy to get max Heat = 100 max Heat / 1.4 Heat per Energy = 72 time to get max Heat = (72 Energy - Fire Blast cost) / cost per second Old single: (72 - 5) / 12.5 = 5.36s New single: (72 - 10) / 7.5 = 8.27s Old double: (72 - 10) / 25 = 2.48s New double: (72 - 20) / 15 = 3.47s So about 3s and 1s more field time for single and double respectively, assuming a playstyle of continuously holding Skill and triggering Fire Blast once. If Fire Blast can be spammed quickly then field time might be lower.