Having spent an entire career applying logical thought in a problem solving role, trying to understand what is going on here has me beaten.
Before G.INP I had almost 4 years of fastpath connection with 70 to 80 error seconds per day. The broadcom based HG612 gave a consistently higher sync than anything Lantiq based, by about 3 Mb or so.
G.INP is rolled out and the HG612 takes a 3Mb hit, whilst Lantiq based kit does the opposite. Still on fastpath, but now zero error seconds. So I switch from having the HG612 in front of the Draytek to using the VDSL connection on the Draytek itself. Net result is more or less same speed but lose the 70 to 80 error secs - so really no difference either way.
Now I see all these reports of people with high error rates losing G.INP that has greatly improved their connection, whilst G.INP remains steadfastly active on my line when really it isn't doing much at all.
Go figure!