Not connected it all up yet but initially it will be outside in a cool place, so the breeze maybe enough. It may work with convection as you say but convection, (similar to heatpipes), requires a significant difference between the water temp and ambient to start to work, so I gesss it would probably run about 5C higher temps overall compared to just the most minimal airflow.
Just done some barb reaming and these pics show the difference. Both are 5/8" with 3/8"bsp thread, (near as damn it the same as NPT), The original having a 10mm ID & the reamed a 13mm ID. This may not sound a significant increase but it virtually doubles the hole volume. I included a standard 1/2" barb in the image for size also.
