[Sephiroth]

TH

I think you need to define the term "quality" as you see it for the purpose of the question.

I can certainly answer the question as regards upload limitations. In simple terms (I can explain it in much greater depth), the upstream operates in a noisy-ish (susceptible to noise) part of the frequency spectrum. Accordingly the packing density of upstream data is lower than downstream so that the potholes created by noise can be corrected at the CMTS. The frequency range is prescribed by international bodies.

The only way of increasing upload speed within these limitations is to bond upstream channels.