I think you want all the surfaces to be as smooth as possible. Modern Performance says that Extrude Honing makes everything "glass smooth." The smoother it is, the less restriction you have.

I was unable to go deep into the mani with my flex attachment, but after I ground it all out to the shape I wanted, I used a 120grit flap wheel to smooth it out.

With a P&P TB and intake mani you should get, what, about 5-8hp? I'm not sure. I did too many other mods at the time to tell. :P