In other words, if I have 1000 Quantity, then Mine 100, and gain an extra 5 from the current Mining Production Tech, is the Quantity afterwards 900 or 895?
I've also wondered this. Speed or efficiency? I really don't know for sure but I think it is speed rather than efficiency, so you are just mining the same total amount of minerals faster, not getting any extra from the increased tech. You will mine more per year but run out faster, gaining no increase to total minerals mined at the end.
In my opinion higher tech should get you a slightly larger amount of minerals from the total available. But there might be a good balance reason for not doing this. Or I might be wrong and it does give you a greater yield overall.