Soffit trap now done, and installed. 25cm*20cm*100cm. I measured it to fit between a shelf and the ceiling corner, so it was easy to install.
I still haven't gotten around to measuring on REW, although I have re-run ARC.
Installing the two panels either side of the screen, the rear bottom corner bass trap and the soffit trap caused a
massive improvement in overall SQ. Imaging, bass clarity and punch are far better.
The most surprising thing is that the rears now sound more alive and integrated - my bet is that's down to reduced comb filtering. Most pleasingly, the bass from my two USC15s is now almost where I'd say it couldn't get much better. I went through all my go-to demo scenes, and each sounded better than I've ever heard it. The John Wick Red Circle scene (cliche alert) made my jaw drop.
To put this in context, in line with the spirit of the thread's subject, I've gone through the following pre pros (or AVRs used as such) in the past year or so: Marantz AV7702; Arcam AVR550; Marantz AV8802A; Anthem AVM60; and Anthem MRX720. I chopped and changed (without making much of a loss) in the quest for improvement. The AVM60 was the best but the MRX720 was more or less equal. The biggest leap was the move from Audyssey to ARC. I'd say these recent room treatments have now produced a bigger leap.
I still wouldn't say room treatments are the most important system upgrades - a good sub (ideally more than one) makes the biggest improvement in my view - but I'm now 100% sold on the idea that good room treatments make a bigger improvement than changes between electronics, and are far more wallet-friendly.