It seems to very dependent on the individual line.
My ECI line is long, overhead, rural...and banded at 15000K downstream with low-interleaving. (It used to be banded at 11400, but after a concerted effort to remove EMI sources it went up to 13096 and then 15000...but has been stuck there ever since as I've never had a long enough run of "green" days at 15000).
Broadcom modems (Billion and Zyxel) will hit the 15000 limit whenever they're resynced, and report a line attenuation of around 28 dB. Lantiq modems (HH5a and Draytek) won't get anywhere near that at default SNRM - on a very good day they used to sync at 11400-ish, but more recently around 10700 and report attenuation of 33dB. Looking at the bitloading, the Lantiq modems load far fewer bits at higher tones, consistent with the reported attenuation. Also, the Lantiqs have the advantage of being "allowed" to use all tones from 33 up to the point where there's insufficient SNR to load any more bits. The Broadcoms on the other hand are never allowed to use tones 33-66, and stop at around 755-789 (varies over time) even though the plots suggest that bits could be loaded right up to the top of D1 (i.e. tone 857). I've no idea why this is the case - my guess is the DSLAM is deciding not to allow the modem to use these tones because it doesn't need to in order to hit the banded speed at target SNRM - maybe one day I'll get enough "green" days at 15000 for the banding to be raised so I can see what happens then!
Tweaking the SNRM on the Draytek can get the sync up to a little over 14000, but error rates go through the roof.
Upstream, Broadcom modems never sync at much more than 500 (yep, that's 500) whereas the Lantiqs will always sync a little over 600.