Kinda hard to answer without knowing much anything else about what you have going on.
If the heads are the bottleneck, bowl blending will help “more”, if they are not the bottleneck, it will help “less”. More would be 20 hp, less would be 5 hp.
I did a head swap on my 400 hp-ish street car. 440, 3.23, stock converter, 0.455” lift cam, exhaust manifolds. I swapped from really good set of competition valve grind 915 heads (no porting) to a professionally ported 915s. Both sets of heads were done by the same guy. The ported 915 flowed 260-ish @ 0.500. I would guess the unported heads flowed 220-ish. The car picked up 2.5 mph, or about 25 hp. Of course the non-ported heads probably had 20,000 miles on them. If they would have been “fresh”, the difference would have been less I’m sure.
If your heads flow 220 now, i’d guess they will flow maybe 235 after bowl blending. This means you gain would be less than mine.
Better head flow will help everywhere in the rpm range.
Finally, on a stockish motor, I’d take a really good competition valve grind over bowl blended marginal valve grind heads.