Since it is still within the 3-5 business days for upgrade order completion, I guess they will continue to slowly inch up my speed and improve my line. Starting to make me really think that I should've gone for the 6.0/768 tier, since it looks like they might even be able to achieve acceptable levels for that.
This morning, the sync speed jumped to 2496/512 (from 1984/512). The latest DMT Spectrum plot gives an estimated downstream of 4044 (with estimated upstream at 818). And, cacti shows a slight improvement in SN margin, with no change in line attenuation.
Perhaps I'll revisit 6.0/768 in a couple of months.