I don't know the answer. But I don't mind guessing. And this assumes
that MAF is added in proportion to maintain the same equivalence
ratio.

I will guess that stock fueling is at a point of 70% diminished
return, meaning that the last mm^3 gets about 70% of the HP of the
first mm^3.

If 2000 mm^3 gets 300 HP, then 2500 mm^3 gets: ((500/2000)*1.6*300)
+300=52+300=352

Just a guess.