There are without doubt some lines that perform better on adsl1 than adsl2+ - usually the longer ones.
adsl2+ opens up another 256 sub-channels available for use (why your atten will often increase), but these additional channels could be subject to noise interference, and if your line is too long to make real use of the sufficient bits loaded in the additional tones, then in a few cases the line is worse off.
adsl2+ does have some benefits though in that 1 bit per sub-channel can be loaded compared to a minimum of 2 on adsl1 - hence why a min 3dB SNRM rather than 6dB of adsl1.
adsl2+ also uses better RS algorithms - which in turn means less overhead and more useful data can be carried.
Generally speaking (very generally and not to be taken as true in all cases), but lines that sync at less than 6Mb may not always fair better on adsl2+. Anything 6-8Mb is suck it and see, but usually will see a bit of improvement.
ADSL2 though could be beneficial, as adsl2 has the better algorithm and RScode words, but still uses the same amount of sub-channels as ADSL1. If your line cant sync at more than 10Mb then adsl2
could give you the best of both worlds in that its not opening up the additional channels.
