just got one of these cat 5 rj11 cables replaced the standard cable that came with router and i saw a sync increase from 3424 to 3616 all in all £2.99 well spent
I've used one for a while and, like you, saw a replicable improvement in sync speed (we sync at similar rates...). Others have seen no benefit - possibly their effect is most detectable on longer lines?
my attenuation is 49, so not really that long, i was syncing at 4100 then asked to be put on fast mode , btw then upped my noise margin to 15, hence the lower sync