Found this, below .....
'Modems that do not support (A)ELEM will adversely affect those that do. This is due to the incorrect upstream power back off being applied'.
Interesting.
I was commenting in a thread over on TBB, where a user had some latency problems after G.INP activation. This was a line that was sync'ed at 80/20, with G.INP enabled, but had attainable of 95/45 - a notably high upstream attainable speed.
When compared against my line (currently 107/35 attainable with G.INP), I could see his line had slightly higher attenuation, but was otherwise configured identically for all G.INP settings; it looked like this was NOT the cause of the issue. However, the power values on the upstream were huge in comparison - my upstream power was -8dBm, his was +8dBm.
I remember commenting that it looked like the UPBO wasn't working right, and that perhaps this was having some knock-on impact to latency (but I haven't a clue why that would be).