The generally accepted % HP increases are approximately 5-7% for a 3 angle valve job alone. Blend the valve job into the bowls, straighten the pushrod pinch, smooth the shortside radius and mininimize the guides, blend the pockets and a 15% increase in flow is pretty easy to achieve. Flow is roughly proportional to HP so that would also translate into 15% more potential HP.
Larger intake valves are worth a 3% flow increase (per GSCA test), but having the stock valves back cut can help them flow better, especially at low lifts (some machine shops do this for very cheap or free)
Personally, I wouldn't "gasket match", but would match the actual intake to the head to make sure there was a smooth transition. From a flow standpoint, its good to remove material at the top of the port, but at the bottom of the port, removing material can actually hurt flow. There is a restriction if there is mismatch, and I only would try to remove material that presents a restriction. Rather than spending time "gasket matching", carefully smooth out the pushrod pinch and the port "ceiling".
