I have been looking at this for the past month now and the situation is this (All figures captured after being synced for several days) :-
Average figures on ADSL(1)
Sync: 3552, 448 Attn: 55.0, 29.8 SNRM: 2.9, 20.0
BT Profile: 3000, Download speed achieved was - 2.82 Mbps
Average figures on ADSL2
Sync: 4098, 446 Attn: 51.5, 29.8 SNRM: 2.5, 14.0
BT Profile: 3500, Download speed achieved was - 3.31 Mbps
Average figures on ADSL2+
Sync: 4112, 448 Attn: 56.0, 29.8 SNRM: 2.9, 8.3
BT Profile: 3500, Download speed achieved was - 3.32 Mbps
The downstream attenuation decreased by 3.5 on ADSL2, but increased by 1.0 on ADSL2+
The average downstream SNRM decreased by 0.4 on ADSL2, but was the same on ADSL2+
The average upstream SNRM decreased by 6.0 on ADSL2, but decreased by 11.7 on ADSL2+
Throughput is the same on ADSL2 and ADSL2+. They have both increased due to the higher BT profile.
In all cases, the SNRM was initially forced down to around 3.5 on re-boot using DMT on a DSL2640B router.
SNRM is fairly stable at 2.5 during the day and does go down to around 1.5 overnight.
Router has now been synced on ADSL2 at 4098 for 15 days.
I decided not to use ADSL2+ as I would probably never use the higher frequencies.
On ADSL(1), apart from the fact that I could never achieve a sync of greater than around 3500, it would probably re-boot once per week.
Definitely seems more stable on a line of approx. 4kms.