If the walls of the head and the manifold match, then all you really need is the proper size gasket so that the transitions are smooth. When I was working on engines back in the '70-80s, we used RTV - run a small bead around the edges of the gasket while it is in place on the head or manifold, allow to set, and trim with an Exacto knife. Don't know if that will work on modern engines. Didn't have the too-big-gasket-hole problem on the exhaust side - they were nearly always too small and needed to be trimmed.
Back in The Good Old Days, you could get better flow with a good port-matching/relieving/polishing job. As you mentioned, the manufacturers are now doing a pretty good job of this at the factory, so gains will be small. Not worth the time and effort unless you track the car, IMNSHO.
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