Infinity

How sure of you that the initial 40 second duration is buffed to 66 seconds with max Magic Mastery? The way I read the skills, the skill duration is buffed to 60 seconds. To compensate (and this is something else I believe you may be incorrect about, though if you show proper evidence to the contrary my contention is moot), the skill initially holds a +10% damage buff immediately, which then additively stacks every 4 seconds to +100% at the end:

 Spoiler

Time in seconds: +% added damage

0-4: +10% damage
5-8: +20% damage
9-12: +30% damage
13-16: +40% damage
17-20: +50% damage
21-24: +60% damage
25-28: +70% damage
29-32: +80% damage
33-36: +90% damage
37-40: +100% damage

With max Magic Mastery, the table would appear as follows, with intervals extended to 6 seconds to match the +50% increased buff duration of one minute:

 Spoiler

Time in seconds: +% added damage

0-6: +10% damage
7-12: +20% damage
13-18: +30% damage
19-24: +40% damage
25-30: +50% damage
31-36: +60% damage
37-42: +70% damage
43-48: +80% damage
49-54: +90% damage
55-60: +100% damage

Again, I could be completely wrong, but reading the skills literally and applying the concepts throughout, the above tables make more sense than some skill lasting for 66 seconds when, with its duration multiplied by 1.5, it should only last for 60.