Great find. Having owned a Gen5 SS 1LE and now a Gen6 SS 1LE, I've made it a point of maintenance to clean all the junk that builds up below and around the radiator/aux coolers.
Two things I noticed in your description of the fluid flow path. The line that runs up the passenger side from the diff, according to a parts diagram that I pulled from a '23 A10 ZLE, it connects to the passenger side of the horizontal aux trans cooler(#16 in the below pic), not the upper radiator port. This duplicates the diagram of my A10 equipped SS 1LE.
Also, the fluid path
may be reversed. I base this on a few documents of noted fluid flow and my own experience. Long story short, even to this day with my own findings during my ATF fluid change procedure, I have my doubts with what I have observed are correct, and this would point to you being correct. While the documents may be wrong, they do almost seem counterintuitive. It is possible I have missed something in this understanding because I have not confirmed myself which trans port at the thermostat/bypass block daylights fluid back into the pan. In the second pic, it looks like cool fluid travels down the passenger side first, to the diff, picks up heat, then travels down the driver's side. If this is the case, yeah, it is odd. I would think getting the coolest fluid to the trans would be priority #1. The last pic is from Improved Racing's GM trans thermostat adapter instructions. It shows the rearward port is the outlet port, sending fluid to the cooler first, which fluid would then go to the diff and finally back to the trans.
Trans fluid flow pat starting from:
the pickup in the sump
thru trans circuits/converter
out to coolers
to diff
back to trans
??
https://www.improvedracing.com/6l-8l...hermostat.html
#fjb
