Which aluminium extrusion for LED strips has better cooling?
If there is no fan, natural convection will dominate and the airflow will be driven by the hot surface temperature, starting at the surface. Therefore, especially in case of up-side-down there will be no air flow blocking. The surface looks like being increased be about 10%, so it is a good assumption that also the surface temperature will be reduced by roughly this amount. Radiation will increase the effect slightly if surface temperature is well above room temperature.
In case of dominating natural convection small surface changes can have a measurable impact of cooling performance. With forced convection (fan) this is usually not so much the case.