I highly advise against polishing intake runners both in the intake and the head. The problem with polishing them is that it allows the fuel air mixture to become unsuspended, costing a lot of HP. What you want is a florentine finish. Something to do with laminar air flow, and since Im not an aeronautical engineer, I wont try to explain it. What I do understand is that you need a certain amount of roughness in the surface of the runners and ports to keep the fuel air mixture suspended.
Polishing on the exhaust port side, however, is an excellent idea.
besides not needing the rough surface to keep the fuel air mix together, polishing the exhaust side reduces the ability of carbon etc to cling to the runners causing restriction.
I heard a lot of hype about extrusion several years ago, but the cost was so prohibitive, compared to any benefits. I think it kind of died on the vine.
You dont want to just go in and start cutting down the divider in a dual plane, indiscriminately.
There is an outfit in Wisconsin called Brezinski that specializes in doing this with factory steel intakes for stock car racing, where stock intake manifolds are required. They also do a lot of other internal work in the intakes.
Ive talked to them a few times. It takes a lot of time and work to get that just right.
Take too much and you mess up a manifold, take too little and youve done nothing.
Plenum chambers are a very finnicky area to play around in.