Before you found the fix, I read your posts and it sounded like you thought the bike was running very rich. I thought the opposite, that the problem you described was actually momentary fuel starvation, caused by a sudden drop in vacuum when you opened the throttle any quicker than the set up could handle. Pods will do that to CV's.
I made a reply suggesting to richen the pilot circuit or jet needle circuit, sent the reply, then realized I missed two pages of your thread and you had found the fix. I deleted that post.
If your nylon ring is similar to the ones used in the VM series carbs, it is quite thick and does force the jet needle downward and that leans the mixture at 1/5 to approx' 3/4 throttle. Removing it would richen the mixture by more than one needle clip position...much richer. On my VM's, it doesn't cause any issues, in fact, it needs to be installed or there will be metal to metal contact.
I'm just glad you're happy with your bike now. Owning these old bikes can be frustrating when you factor in age, wear, previous owners, etc. At least with the GSR you're not alone. We've all been there. Have fun!