Supra stuff:

Weird as it seems to me I wonder if there's a small boost leak that opens up, closes, opens up, closes, and that causes the variable power output and lack of overall power. I'd think over time it'd get worse, and/or the offending leak would open up entirely and the car would be super slow. I went in to replace the intercooler hoses today but the silicone kit that came in the car didn't have all the right size/shape hoses. Good thing I hadn't removed the old ones yet. I looked it up and this is exactly what I have. I can't figure out why half of those hoses are there. Two of them, the ones on the drivers side, are fairly direct fitment. There are two additional hoses in the stock system, three if you count the one at the top of the engine bay, and none of the other UPGR8 hoses come anywhere close to matching. It's as if the UPGR8 kit is made for an aftermarket IC with non-stock piping. But nothing says it is.

Another thought is the wastegate. The #1 Turbo WG controls overall boost. I tested the actuator and it seems to be working although I don't know exactly how it should be. The manual says to apply 17.4psi to the actuator and move the rod to make sure the WG opens. I can't reach the rod. I can see it move a little bit right around 17.5-18.0 psi, and a little bit further at 20psi. Probably 1/4" in total. It releases with no bind when I bleed the pressure away (I did buy the Mityvac that supposedly breaks all the time...). So this test is pretty inconclusive to me.

I've eliminated some things in the system as problems, and there doesn't seem to be too many things left that would control overall boost. A lot of the complexity in the system lies in the hand-off between turbos, and that doesn't appear to be where the primary problem is because it runs worst under full boost when both turbos are supplying pressure.