My first Mewtwo
Answers
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However, without checking my breakpoint spreadsheet, I'd guess that 15 atk IV is not too important and only with Psycho Cut. Mewtwo has high base attack, so the atk IV adds a smaller relative amount. The higher power of Confusion is also less likely to hit a critical breakpoint, and even if it does miss, the breakpoints are about 5% for Confusion vs 20% for Psycho Cut.
But if you check the table, you'll see that for the higher power (or base damage) attacks, the breakpoints come closer together. I've powered my 14 attack one with Confusion up to 31, which is a Machamp breakpoint. There's two more before I reach level 40.
Psycho Cut, on the other hand, has a single important breakpoint around level 26 (still vs. Machamp), and then it's maxed out. You can still increase charged attack damage, but fast attack is done.