But technically aren't you really increasing your dose anyways.
it is possible that brand name may have a bigger serving of oxy in each pill than the generics you are correct as you know with the generics there "can" be a difference in the amount per pill and this is actually approved by the FDA, I think it's around what you said like up to 20% less, but I;m not sure.
But remember it can also be the same or different every time, my point being that probably every batch of pills the generic companies make I;m sure can vary from one manufacture lot to the next. And you could be taking the correct dose for all you know, there is no way to asses wether your pill has less oxy than the brand because obviously the FDA allows for variance all the way to the top limit. The only way you could know for sure is buying the brand name and seeing what happens.
SO if you want to try a brand name you are right there is a chance it could have some more of the med.