Hi again FC,
Samknows says there is no date for BT's 21CN upgrade but there is a good chance that the TalkTalk service is already ADSL2+ enabled.
You should obviously check this with TT.
IF SO, the TT service might be a little faster on data speed but contention might slow the actual throughput down at busy times.
There are really too many unknowns at present to make definitive statements.
Once you have the line stats if the vendor obliges, the Kitz max_speed estimator will probably be the best guess.
Kind regards,
Walter