Great job! I’ve read your experiment a couple of times, and liked the way the job turned out.

I’d just like to say a few words in favor of the Lightweight RD

I’m not an Sram owner but understand that the Sram road derailleurs only have one pivot spring. I.e. the upper pivot spring, the one at the bolt for mounting the RD onto the hanger, is missing on the Red, and present on shimano and campy derailleurs. Not that there’s anything wrong with it, Red seems to work fine woithout one.

Just saying you need to compare apples to apples.
Is the Lightweight RD also a single spring one?
I think not, judging by the pictures that show a “hanger stopping tab” (is it called like that?) at around 3 o’clock of the RD mounting bolt. (far too much forward for a RD with an unsprung upper pivot.) The Red one , for instance, has that tab at aorund 6 o’clock compared to the same bolt, if positioned the same way as the lightweight pictured above…

I think some builders opt for such 2 spring construction for smoother shifting? not sure though. Anyway I guess the Lightwieght RD has some extra structural elements compared to the RED.
Remove those and the Lightweight will drop some more weight.. and -might- still function properly maybe less smooth.. or not at all any more