In addition to this advice I would suggest to, if possible within that subset, get a device that is currently or was at some point given out by the provider.
While I strongly believe in experimentation and optimizing hardware myself, with leased lines it is very possible that one day you will have to rely on the ISP's support to fix the line. Running a device on your end with which the ISP is familiar will probably not change anything regarding the technical fault, but it will most certainly simplify the process on their end. (Having had a few troubles with my line in the past, it really got very tiresome to explain a "nonstandard" device to customer support, always fighting an uphill battle to convince them that my device is not the problem. Once to the point where a technician at my place was confused that my device was not "blinking correctly.")
(Also, as an added benefit, ISP devices are given out by the thousands and are really cheap to get second-hand.)
Apparently, there are issues with vectoring support in the current openwrt Lantiq driver, although the poster of the thread below who discovered it, is working on a fix.
K I tried with the ZTE ZXHN H188A which is supposed to have a broadcom chip and these are my current stats:
Actual Rate(Up/Down): 5119/40960 kbps
Attainable Rate(Up/Down): 48324/120995 kbps
Noise Margin(Up/Down): 31/23.7 dB
Line Attenuation(Up/Down): 0/4.2 dB
With small change in the Max Attainable rate but the attenuation and noise have have the most change. The downstream noise margin increased by ~3 db but attenuation increased by ~2.2 db as for the upstream the noise is pretty much the same but the attenuation dropped to 0.