If they were able to increase the QAM from 256 to 1024 then each channel could handle 69 Mbps. Igni points out that the infrastructure must be capable of handling the higher SNR required for 1024QAM to operate reliably.
There is a useful paper
here. Please note that it refers to DOCSIS not EURODOCSIS; so the throughput quoted can be increased to 8/6 of the value quoted in the paper.