A bit of further tuning and it seems that setting a downstream rate of 96% of 0.884434 of sync rate is the best for performance, according to the AA speedtester, at least. The 0.884434 figure = 1500 / ( 32 * 53 ) is for ATM 32 ATM cells, assuming in my case PPPoEoA rfc2684 VC-MUX with 32 bytes of one-off overhead per packet on top of a 1500 byte IP PDU and then the ATM per-cell bloat.
Increasing the rate above 96% seems to have a negative effect. A bit surprising to me. I just thought it would hit a wall and there would be no improvement, not a reduction in performance.
With those settings the AA speedtester gives a reading of ~9.8 Mbps downstream, occasionally nearly 10Mbps, and 1.53Mbps upstream - whatever that is supposed to be a measure of - it could be claimed to be maybe TCP payload, or maybe IP PDU rate.
Perhaps somehow those figures that I am getting for sync rate are a bit optimistic, or I have forgotten some other overheads or something, but it really seems as if perhaps the 0.88 limit is higher than the reality possible, or the sync rate quoted is too high.
Or maybe there is something else going on that is concerned with optimising the effect of interactions with the behaviour of TCP, or the behaviour of the speedtester.
Other practical tests: IP PDU rate when doing downloads from Amazon is 9.6-9.8 Mbps very roughly, so superb. The benefit here in getting the additional line is quite startling.
Sometimes Netflix goes very fast indeed when downloading a series of 45 min programmes but on other occasions for reasons that are unclear Netflix IP PDU rate can be up and down, dropping as low as 6 Mbps then up to 8 or 9 Mbps and basically all over the place sometimes so don’t know what they are up to but it does rather seems they have certain problems. How hard is it to simply download a file? I thought we had this one sorted about 40 years ago. They seem to keep introducing bugs then fixing them and then going round the cycle again. Netflix need to have a change freeze while they just observe, measure and test, and then they either carefully fix core bugs, or else get ruthless, throwing bad code out and replacing it from scratch.