Stew, I don't know exhaust flow, I mostly know blood flow, but even with steam flow (I am working on a PhD in Biomedical Engineering, and am former mechanical operator for naval nuclear ships) piping directional changes should be smooth bends to limit laminar flow disturbance. Disturbance causes back pressure, and while some is necessary, I don't like the angled directional changes of the sandy spares set. I have not tried them, but I'm not gonna, just due to that...FWIW
Thanks for the tip, I wonder if they ship to the US?
As for the flow, I learned that theory a long time ago. Not saying it is wrong, but I do not see any thing in this design that does nor occur in the stock exhaust. The nature of the design in the V-4. I was talking to an exhaust expert once, about your exact concern. The most important angle is the one coming out of the head. Then went off about volume of the pipe, expansion rates, back pressure etc.
So everything you say is accurate it leaves off the fact that packaging the exhaust to fit that rule, would be a challange even Honda did not pursue.