I've never given a thought to power increase in connection with the 3 angles. # angles are needed to establish the correct width of contact between the valve head and seat. How far you grind the top and bottom angle will determine where the seat contact area meets the valve. I want the contact area just slightly out from the middle of the valve, as the valve will expand slightly when it gets hot.
There was an International tractor shop near me years ago that only ground one angle and never cleaned the grindings out. I would never have had them do my work. Maybe they worked that way to get repeat business.
