We have a winner!
Turns out the cats are clogged now. I drilled three 5/16" holes in front of each rear cat and suddenly the car revvs up again nice and smooth.
I'm gonna go get the rear cats swapped for off the shelf universal cats at a shop later.
I think this whole deal was a multitude of issues. Most of all, I believe that maybe my valve stem seals fouled the cats with oil over time (I know #2 had pretty nasty plugs when I first pulled them), the tune maybe wasn't fully dialed in, and leaky injectors added onto that and caused the weak engine performance, as well as a cam synchronizer that was probably on the way out.
After I swapped the injectors the car suddenly had low load power again, I could keep it cruising at 2k rpm at 40-50mph in 3rd with almost no throttle. That tells me they were done. But I think in the process I have clogged up the cats so much that they are done now.