hey jammer, I don't know what the flaws would be really if any. The gains would be much smoother airflow around the seat and into the combustion chamber. I do know that the angles that are cut into make a difference in how it flows. The closer you can get to a full radius on the seat the better the airflow past the valve.
Here is a link that explains it on the cvh/spi head mostly but it's the same principle for all motors that use valves.
The 3 angle valve seat cut is nothing new. Its been around since dirt. So if their trying to sell you something special thats a joke. Im sure all machine shops use the 3 angle cut as standard proceedure. The current state of the art is a 5 angle cut. For all but the most extreme racing conditions the 3 angle is fine. You wouldnt be able tell any difference between the two. The price quote you stated for the work seems high I would shop around.