Cutting the valve seat is different to an angle grind on the valve itself.
But yes, cutting the vlave seat right on the edge of the valve increases performance as you said by increase valve size, but decreases realiability. In one of my race cars, we had to re lap the valves every 3000-4000 miles with this.
The valve angle grid job smooths the transition of the flow of air from the intake to the combustion chamber, by smoothing the transition from the valve stem to the face. imagine it like a slide. the smoother the slide, the more curvature the faster you will go compared to a rough slide.
I have had a 7 angle grind done before, unless you are going for silly hp, then details like this can be overlooked, a 3 angle is a good option.