Just from my experience porting intake manifolds a good designed single plane intake has a few basic improvements possible from porting. Main one is keeping the fuel and air mixed not letting fuel puddle out of the air stream, that is usually accomplished by texturing the intake runners especially the floor of the plenum. Second improvement you may see is blending to stop turbulence where the air has to turn to go down the runners. Uless there is casting irregularities most intakes are pretty good in this respect. finally, a good intake runner has some slight taper built into to increase velocity of the air into the head. If you are not careful in porting a runner and mess with this taper you can screw the intake, air hates to speed up and then slow down, the volume of the runners including in the head should not increase and decrease in cross sectional area. With complicated shapes like heads and runners this is tricky sometimes.
On dual plane intakes I have done some work where class rules dictate the use of a dual plane intake, the flow through the runners is never the same with a dual plane, again texturing works to keep fuel suspended, but you can almost never get the flow equalized between runners only improve it. Increasing the plenum area is the most common thing to do. Illustration is of a perfect port if nothing was in the way to make it.