My sincere apologies, you will know far more about it than I ever will. :-) I'm just surprised at your sync speed given your attenuation figure, which of course takes into account thin wiring. Perhaps your wiring and green cab is rubbish in some other way. Depending on how you feel about the use of terms, do you include crosstalk under the heading of 'noise'? According to Black Sheep iirc, my wiring is the fattest copper, for most of the length at least.
If your noise is ok, then that either means that mine is ridiculously good, or it's to do with modems, filters, cables etc. I don't know what my crosstalk is like. I have no filter at all (no phone), good ultra-short dsl cable, no house-internal wiring, triple-filtered mains supply to the modems just in case and aggressive modems, I believe. So maybe the difference is in there. I've spent a lot of time over the years comparison testing-kit and swapping things over, including making some 'improvements' that are probably pseudo-science and not really relevant but can't hurt.