I would be grateful for any comments (praise or brickbats) on experience with the unit with latest firmware installed.
I have one last thing to try which is enabling the correct DSCP mapping on the WAN port of our router as I spot that they use the DSCP bits to determine what's voice and data.
Item: I would:
1) Set the VDSL Profile to 17a Multi-mode - this seemed to have an impact for me.
2) Disable Seamless Rate Adaption (SRA) if your line has been pretty stable until now.
If you still don't get where you want to be, then you can try tweaking the stability adjustment - but I would use this with caution as it can cause issues and there are others on here who can give you far better advice than I on this!
FWIW mine is set to +8DB and the Gain Adjustment is set to "Stable" as recommended by an Asus tech on my Slingshot line.
Any comments made are personal opinion and do not reflect directly on the position my current or past employers may have.