It’s actually very easy to calculate the damage on Starsurge. The base damage of the ability is 162-198 (it’s a very large range). I currently have 102 arcane damage and I’m nearly BiS also. The maximum arcane dmg a balance druid can have in this phase is ~123. So that means the damage range of a BIS Starsurge is 285-321. You could throw on a +9 Arcane cloak and both of those numbers would go up by 9.
So I agree 267 is low, but it’s not that far off. Most players don’t have the epic staff, lol, so practically speaking 267 low end isn’t that crazy.