What it's "worth" will vary with every individuals opinion. Does it make a difference? Yes, the degree will vary with the deign as JohnRR suggests, as well as with your other parts in the combo, the benifits will vary depending on where the bottlenecks are in your combo. I have not done flow tests but have had a set of 340 manifolds done, as compared to stck they are not noticabley larger (hogged out, etc.) but they are extremely smooth and clean inside which I'm sure affects the flow in a positive mannor. My guess is 5-10HP gain overall, not much but when every bit counts a gain is a gain.