Experiencing very poor service quality - what are acceptable SNRs/power levels?

For a while now I have been tracking my download/upload using Cox's speedtester. I am signed up for the 200/10 plan. At peak times I understand this can vary significantly but on off-peak I should expect near-ideal. Unfortunately, regardless of the time of day I am seeing anything between 10 Mbps and 50 Mbps regularly when hard wired. My cable lengths (cat 6a) from modem -> switch are short (< 20 feet) so I am not getting attenuation along my lines.

Below is the report off my Arris SB6183.

Channel Lock Status Modulation Channel ID Frequency Power SNR Corrected Uncorrectables 1 Locked QAM256 37 381000000 Hz 5.5 dBmV 42.1 dB 0 0 2 Locked QAM256 33 357000000 Hz 5.3 dBmV 41.4 dB 0 0 3 Locked QAM256 34 363000000 Hz 5.3 dBmV 42.4 dB 0 0 4 Locked QAM256 35 369000000 Hz 5.4 dBmV 42.5 dB 0 0 5 Locked QAM256 36 375000000 Hz 5.5 dBmV 42.1 dB 0 0 6 Locked QAM256 38 387000000 Hz 5.6 dBmV 42.2 dB 0 0 7 Locked QAM256 39 393000000 Hz 5.7 dBmV 42.4 dB 0 0 8 Locked QAM256 40 399000000 Hz 5.7 dBmV 42.4 dB 0 0 9 Locked QAM256 41 405000000 Hz 5.6 dBmV 42.3 dB 0 0 10 Locked QAM256 42 411000000 Hz 5.6 dBmV 42.3 dB 0 0 11 Locked QAM256 43 417000000 Hz 5.5 dBmV 42.3 dB 0 0 12 Locked QAM256 44 423000000 Hz 5.5 dBmV 42.4 dB 0 0 13 Locked QAM256 45 429000000 Hz 5.5 dBmV 42.4 dB 0 0 14 Locked QAM256 46 435000000 Hz 5.4 dBmV 42.3 dB 0 0 15 Locked QAM256 47 441000000 Hz 5.3 dBmV 42.2 dB 0 0 16 Locked QAM256 48 447000000 Hz 5.4 dBmV 42.3 dB 0 0

According to Arris these are in spec. Upload is always near-service-level so I am leaving that one out. What SNRs should I shoot for? Attenuators are cheap and I'm happy to put a forward attenuator on to bring these down 3-6 dB if necessary. I am not experiencing any correctables nor uncorrectables so I am sort of left without an easy excuse for why this isn't working.

I am hoping I can get some help here because Cox wants to charge me $5 more per month for "advanced technical support" to solve this service level problem. I am not even getting the speed I am paying for so understandably I am not interested...

Hoping to get this resolved so I don't have to switch to century link. If I only peak at 50 Mbps down I may as well switch to DSL and save $100/mo.