If you are getting 6.5Mbps on ADSL then I would expect to see some kind of improvement on ADSL 2+ but you won't get the full 24Mbps of course, maybe 8-10Mbps. I think Andy made a good suggestion of checking the firmware and options to make sure the modem knows to use it. Also how do you know you are on an ADSL2+ DSLAM ???

For the best results get yourself on of those new fangled front plate filters BT are selling for £10, they cut the ringing circuit out which should give you a boost in speed if you have lots of extensions around your house.

21CN hasn't been rolled out yet, and won't really increase speeds in itself. 21CN is a major change in the back-haul service and most of us won't really notice the difference day to day. To cut it down to it's very basics they are changing to a fully IP based back haul for both voice and data between the exchanges, as this means a change of equipment us as end users can expect to see some new features but not massive increases beyond what is already possible with the range of DSL services on the market. I should also add if your SP has swapped you onto their own network for voice and data then 21CN won't effect you at all, as it's a BT Wholesale product and will only effect customers who's SP use BTW.

Cheers

Cris.