Well I did finally join the 4 upstream channels club (4 x 64 qam).
According to
this site, thats 6 bits per symbol, and my symbol rate is 5120k
So I guess 5120 x 1024 x 6 is my max upstream bps (31,457,280) = 30Mbps per channel.
Assuming this maths is [now ]correct, four channels allows for 120M upload (five would max at 150M).
*Note: My knowledge of qam and bit rates is limited, so the above could easily be completely wrong.