I don’t agree with there being “no point” but I agree it is “very hard to tell”.
We don’t know the loot tables or how drop rates are affected by treasure multipliers, so we can’t work out your expected yield for finishing certain numbers of rooms. GoW loves randomness, so it’s very likely some Level 8 chests will give you less or equal rewards to level 2 chests in the name of “bad luck”.
So that you got one instance where a higher-level chest gave you fewer rewards than a lower-level chest doesn’t tell me “it’s not worth it”. I got 8 mythic ingots from one Level 8 chest. I have received no more mythic ingots from them. I’ve seen a lot of people get 2, almost as many get 4, and very few claim more than that. So I assume I had really good luck once, and below-average luck since then. Go me.
Today I got more than 100 chaos toenails from one Delve. That’s more than I’ve gotten from all 3 put together on previous days.
It will take a lot of number crunching and/or reverse engineering to figure out the specifics. It will likely work out like most other things in GoW: “If you consider all of the Delves you will ever play, it makes sense to always finish as many rooms as possible. But if you consider small runs of Delves, the differences are miniscule.”
I wouldn’t be surprised, in terms of the 8-mythic-ingots case, to find finishing all the rooms only increased my odds by less than 1%. But if I expect to play 3 Delves daily, that means every 33 days 1% should pay off.
If we were JUST talking gold and souls, then sure. The cutoff would be, “Can I get more of this faster from PvP or Explore?” But when it comes to legendary/mythic ingots, you can’t. Spending an extra 20 minutes for 1% of a mythic ingot is still better than 20 minutes of PvP.