wouldnt temp in/out of wort and water be the same regardless? obviously the highest delta t results in the most heat transfer. maybe i misunderstood the question.
I think the correct answer is that it doesn't matter for heat transfer. The and answer to the diameter issue, I assume, is that's how it works best with the most common way people use it - common hose diameters as an example. Ideally you would flow the most of the wort and cooling water though...
Ginger dominated mine, but that's what I wanted, because I f'ing love it- think store bought ginger beer level of flavor (but no heat). Maybe try half of what I did for a more subtle flavor.