Hey!
The time to depletion should be regardless of productivity modifiers. Essentially, this mod regularly adds up the amount of remaining resources that are in the individual entities of a mining site. The depletion estimation is then based on the change of that amount.
Following that philosophy, I don't see a good use for multiplying all these values with the productivity modifiers. Although that would be fairly trivial to do, the next problem would be individual miner productivity (modules etc), which is very hard to correctly estimate in all cases.
But maybe I'm not seeing possibilities or benefits, so feel free to expand on your idea
