I have dug up a set of release notes for the low-level broadcom driver in the web somewhere. In the bug fixes I saw, I don’t remember noticing anything relating to lowly G.992.3. Is that correct? I didn't know how far back to look as I was not sure what the baseline low-level driver version was.
I found this from cisco detailing the changes from 39
v to 39
x1:
https://www.cisco.com/web/software/282821780/140978/ReleaseNote_A2pv6F039x3.pdfOnly these lines mention G99
2.x
Improve kl0 estimate in G.992.[35]
Fix long training time in G.992.1 against AD72 DSLAM
I'm not sure what these even mean so
If you see no improvement with the 39
x1 driver its probably best to stick with the one zyxel chose to use. I was interested it trying the later driver as I have experienced some weird behaviour with my line and recent firmwares and have G.vector (G.993.5) and there are several mentions to improvement with it. There may be unforeseen consequences to just swapping out the adsl_phy.bin without changing the kernel driver as again from the cisco document:
Improve power saving in DSL PHY by powering down unused line. This feature requires
new xDSL driver A2x026n
With 26n I believe referring to a later kernel driver to the 26a one these zyxel firmware have. So be warned its experimental and probably not advised, but hey, if people dont see and stability issues and see some kind of improvement then why not.