The gap is 1-thou (30um), this isn't challenging machining - you could make the gap 30nm ! (it's a bit more annoying with copper)
Actually some machining marks would probably be good, small surface irregularities will break the boundary layer - like sharks skin and make the air flow mix better.
Agreed. I've made "reference" flat surfaces on aluminum using our Haas VF3-SS vertical milling machine with a good quality shell mill with new inserts and, if you do everything right, you can see yourself on the mirror-like surface that results. This machine is probably the grade of machine you might expect to find in a high-volume production shop.
Actually some machining marks would probably be good, small surface irregularities will break the boundary layer - like sharks skin and make the air flow mix better.