Makes sense that the more gentle curve would have the least losses. I'm wondering if the inner diameters of the fittings are different enough to make a difference.
I once needed to make a tight 180° and I used several 45° copper fittings to do the job. So I am curious how two 45° fittings would perform compared to a single 90° fitting. Also, I believe you have mentioned testing a 5/8" Nylon elbow, maybe you could put that data on the figure for more comparison.
I am curious about your testing methods, is there an earlier post you made that details your procedure?
Nice work on the custom wye. Was it a PITA to make?