Apologies, you just said 'its totally temperamental and needs to be bang on (look at all the unhappy peep in this section).' which made me think you were referring to 100Mb specifically.
There is a wide operating range built into the DOCSIS standards. My own service has worked fine at +17dBmV downstream and 31dBmV upstream, and works fine at +4dBmV / 44dBmV. Each 3dB change is a doubling or halving of signal power - there is extensive tolerance built in.
Of course it doesn't require manned upgrades and the doubling of speeds won't involve engineer visits for every customer, contrary to what a CS guy told me.
The very simple test of power levels is a flood ping with maximum payload, if packets are dropping excessively or are out of order the power levels or signal quality are causing a problem, if they aren't then whatever your tier of service you'll be fine. Errors at the cable level cause slower speeds through packet loss and retransmits, nothing else.
