Speeds are slightly lower
Within a small margin, as expected due to variance in conditions throughout the day.
I do not believe there is much scope for improvement based on the information I have. It looks like the approx. 1.5 Mbps upstream you are getting is the most you can reasonably expect to receive. Furthermore, that speed is snugly within the range on the dslchecker.
It doesn't mean that there is no fault and that you cannot get more, but I gather the line is stable so that would rule out the likelihood of it being a fault. And you've tried different hardware and filters without a budge to the sync speed. I just think that becuase of crosstalk and, as other members have mentioned, PSD masks there is not much scope for an increased upstream.
You could purchase a Broadcom-based modem/CPE with shell access so you can get stats from the device to graph (and show us). You could pick up an old 8800NL (the first version) from the Bay for probably about as low as £20. Though I do not know if it would be worth investing in such a device just for the sake of us telling you there is nothing to improve.