You're using 'optimal' as a synonym for 'acceptable', it seems like. 300 is acceptable if the person running that account knows how to play.
Optimal, though, is always going to be an experienced player with capped CP.
I'm saying optimal b/c we tried several times with a less than 500 player and couldn't make it. I just wondered if Z made it for 600cp on vet hm. But if they didn't know mechanics, then perhaps that's the reason and not the cp level.
This, note that the cp160 player has an problem in that he has not assembled good gear yet.You are asking what CP is optimal. Clearly that would be capped. Of course considering it is allocated appropriately.
I expect you intended to ask the question differently. There are players that with only 160 CP can out dps a great many CP capped players. Skill is the most important factor.
I didn’t start any dungeons of any kind until cp300, but that’s because this forum makes the community look a lot more callous than it actually is.