If the EGR passage was on the floor of the runner, where gas speed and density is the lowest, then the gas would have a tendency to drop a bit and go into a tumble, which would create turbulence and reduce flow.
Given its on the roof where air speed and density is the highest, the pocket will stay pressurized and the exhaust gas will just glide right over it. Its the same as when guys lower the tailgates on their trucks to get better mpg. Its proven that it doesnt work at all. The bed is pressurized and the air just glides over it.
Keep in mind that under WOT, the EGR valve is closed anyway. But if there are sharp edges, THAT can hurt flow. The trailing edge (furthest inside the runner) can be sharp, and should be sharp, but the leading edge (nearest to the outside of the runner..((remember things are backward in the exhaust port)) should have no sharp edges. A rough surface on the walls only accounts for 2-3% of flow losses. The runner walls will be covered in carbon in no time anyway so the walls will be smooth soon. The intake runner, including the intake manifold's runner, should be uniformly lumpy but smooth to the touch....do as accurate of a port job with a burr and then finish it up by hand with red scoth brite to make it smooth. Alot of people hack it up and then try and use sanding rolls to cover their mistakes. Thats bad.
Its like a golf ball. Alot of tiny bumps creates a boundary layer that is like a bunch of tiny swirling air pockets. This creates an almost zero friction surface for the air to travel over. Air travels over a surface, and if it hits a little bump, a wake is created...like the water behind a boat. After that, the farther it travels the bigger it gets until it disrupts the entire flow pattern. But if it hits a bump and kicks the air up, then hits a dip, it drops back down, and then hits another bump and goes into a swirl...and so on and so on. It doesnt grow in thickness. Its stays thin and reduces the un-uniform nature of laminar flow...where speed is highest at the center and slowest toward the wall. This way the air doesnt bog down as much toward the wall. You have more average airflow through the cross section of the runner. People think that a rough surface is good for atomization on carb'd motors only. Well its not just about atomization. Its good for more flow as well. Just a few tips to throw at your head porter, or to apply yourself if you ever do it yourself.
