Valves being the key there. While numbers vary some bench to bench, when I did my research to get my heads done, a standard valve ported RPm would do around 320's. With the oversized intake valve they'd do 340's.
I can't say enough for what a good set of heads does for an engine.
I was just thinking as I was reading the title of this thread on the home page, If you replace small valve, small port, open chamber heads with closed chamber, big valve, big port heads it has got to be a very noticeable improvement.
