The return RF spectrum on cable systems is limited. It's basically 5-42MHz, which is shared with telephony, status monitoring transponders, STB's, other services, etc. Much of the spectrum below 10MHz is too noisy to utilize. It's basically the weakest link in the entire HFC system's capacity.
Unless cablecos undertake a major re-segmentation of their systems, or go Fibre to the Street, you won't see a significant increase in upstream speed.
Roof-top mounted Antennas Direct DB8e & C5, Channel Master 7777 preamp, Siemens surge protection. TiVo Roamio DVR