That's kind've a loaded question, as there is no direct answer, there are other variables at play.
Can a metered orifice above a slightly over sized port regulate gassing with the same size orifice as base barrel ported with that said orifice? I would say yes for most, but not all during a lifespan of that base barrel. There's a lot of variables involved, port distance from the chamber, erosion characteristics of the selected ammunition, etc..
Can a metered orifice above a grossly over sized port regulate gassing with the same orifice size as a barrel ported with that said orifice? It can tend to, but there are additional negatives that tend to add more to reduce a lifespan of the barrel for many. You really want the base barrel porting to be as minimal as possible to regulate gas flow. Even with an additional orifice outside of the barrel, you create a portion of the barrel with an over sized port that allows erosive gasses to wear to a point of being the limiting factor in barrel life.
We pretty much accept throat erosion for what it is overall, but combinations can kill the barrel for its requirements before that because of other things.
It's not really all that hard to port the barrel close to proper requirements, at least for most combinations. Sure, some may not all have the same requirements, but porting for those really isn't all that difficult either for most.
I can see gains with an external orifice to complement some uses to extend barrel life. In some situations, a proper base barrel port will not be complimented in extended barrel life with an external orifice. Either way, you only gain negatives with a larger than required based port.

