Forgot the extra d8 from the Gloomstalker attack, yup. You have 2 extra sneak dice in there, though. So, we're at 10d8+4d6 vs 4d8+8d6 on dice. That's fairly significant even without the stat.
As I said, Gloomstalker hit a bunch of synergies in T2, but this is pretty much peak delta. And is still contingent on the GM allowing surprise.
I currently have a Gloom5/Ass3 in my game. It's nice, but not overwhelming, especially since I don't have a straight Assassin to worry about. And since I like using lots of lower CR foes rather than few higher CR.