Another consideration DV brings up is dynamics. The 30° intake may flow more at low lift values. The Pontiacs were from when valve lifts rarely passed 0.400". The problems as you point out is the flow cone around the valve and valve bounce. DV relates that Cup Car teams have gone to 50° or 55° seats as the valves wedge into the seat without bounce. The steep seat angle centers the valve in the seat for tight valve seal. Those guys do not miss many tricks as the rules are so restrictive, pretty much more than F1. A 30° seat lets the valve sit where it comes to rest and the guide clearance can let the valve get caught up on one side and leak a bit on the other side. On a sedate grocery getter you would be hard pressed to notice.