Gengar Moveset
Not sure how relevant Gengar is in the meta but I have two that I'm considering powering up.
11-12-8 shadow claw/shadowball
14-15-15 hex/shadowball
Answers
Did the 6 attack hurt?
I maxed a 98% Legacy, but I have 2 remaining at level 30 at 11/13/15 and 4/11/15. The 11 attack is already causing me palpitations, never mind the 4 attack.
However I can also work on a 15/10/11 (15 attack!!) from level 20. This might be worth pumping another 75k stardust to be honest.
If you dont bother the additional dust, which is honesttly not that much, Id absolutely go with the 15 Atk one. You hit the final breakpoint vs Mewtwo at L32.5 though, so even your 4 Atk one will reach that, when you power it up to 38 or higher. So yeah, even a 0 atk one can hit the final SC breakpoint when maxed out and deal more damage than a Hex one.
Eventually I would power up the 11 Atk one aswell, as 15 HP are essential for surviving and dishing out one or two more fast moves. 3 SC Gengar should guarantee you the most damage dealt in Mewtewo or Gengar raids. If maxed, the 4 Atk one is viable too, so you cant really go wrong with either of them.
Go for the one with Shadow Claw. Higher damage ceiling despite the worse IVs. There are some quite difficult tier 3 raids where having a Shadow Claw (or multiple) is a huge help in getting ahead of the clock such as Alakazam and Claydol, although both are not available currently.
Plus against legendary raids it's got one of the highest DPS in the game but that's only if it's Shadow Claw, which might help you get an extra damage ball.
Hex still has high DPS but not nearly enough compared to more bulkier counters so there's no point in using it when there's another Pokemon that'll do just as much DPS but won't faint in 10 seconds.
Sadly your answer only comes when there is a differential in CP, as you mentioned it comes at level 24.
Since IV scanning from gymhuntr and similar methods are down, the only ways to find out are:
1. Powering up to the said CP at level 24 to know for sure
2. Figure out the offensive breakpoints and count how many shadow claws you need to beat a low level boss, don't use shadow ball/charge move. Make a video and count manually afterwards. Note, this is only effective if there is a breakpoint between 9a and 14a that one hits and the other doesn't. Use BP calculators here to find out.
3. Figure out the defensive bulkpoint manually by letting a boss hit you and see how many attacks you can take
I think method 1 is the most failsafe, while method 2 is more reliable than method 3 because you can control the exact amount of quick attacks.