The factory valves don't flow very well and are cupped on the chamber side. Aftermarket valves are flat IE nailhead, thus reducing chamber volume by 2-3cc each, and flow better than the OE valves. The factory valve job doesn't flow well either, and is likely in need of freshening up anyway. A 3 angle valve job will increase the flow in the lift range of your cam more so than opening up the ports. It will be money well spent if its in the budget.