I thought I would experiment with etching some wood. I wrote some code to burn a series of lines from 0-30% power in steps of 0.4% and spaced 0.2mm apart (the long box is 0.4mm spacing) The lower left box is done at 150 mm/min and using 5 psi of air, the lower right box is running at 400 mm/min and 5 psi of air, and the top left box is 400 mm/min but no air.
It is hard to see in this photo but the top etch tapers down as the power increases with very little ash, but lots of smoke when etching. The bottom cut looks like a sea sponge, there are many thin slivers of wood sticking up from the base of the etch that are virtually untouched. I took a knife and sliced through a layer of the slivers and underneath we have a sloped etch that is similar to the one without air but deeper.
It may just be the material I am using but I was hoping we would get some sort of a gradient in the burnt wood. However with air we don't get any gradient, and the scorch marks are very black. Without air we get a tiny bit of a gradient, but I think this has more to do with the excess smoke and it settling back onto the surface of the wood.
In short using less air probably improves the look of your engraving a bit, but it does not seem to be essential. Faster is probably better. And you should probably rely on stippling rather than power to convey more than a few shades of color.
Anyway this was just a first stab at etching, I need to run some more experiments for sure.