An opinion from one of the more experienced members would be able to give you a better idea on that.
Since you're still using a standard block/head, increasing the flow will only do so much. You'll be limited by how much fuel/air the engine can burn, all you're doing in making sure it can get the maximum it can use. I would have thought something like 10% extra in some areas of the rev range would be likely.
If it were me, I'd be sourcing a second hand downdraft option ($100) and saving that money for working a new engine.