How much are you lifting your valves off the seats? If you are not lifting the valve above say 0.500" a 30° seat may flow a tad bit better. The problem with the 30° seats is valve seal. A number of the OEMs used 30° seats in the 50's. Valve spring technology did not lend itself to high lift very well. Also materials in cam and lifters were not very compatible to high spring loads and fast ramp rates. Engines then that exceeded 6000RPM were rare for street use. Again we get back to intended use. What works for you may not be what other people expect.