[Christ]

I tend to agree, although without enough experience to say for sure, that you don't need to exit the boundary layer to affect it. I say this because even if you're inside the turbulence, it has to "adjust" it's depth to the new shape. This requires a change in flow patterns, which could be enough to affect flow in general.

My own thought is that level with the edge of the boundary layer is probably ideal, although nearly impossible on an object that sees constant changes in speed/density of the fluid medium.