Dubious BT sales pitch

Just had an unsolicited phone call from BT trying to sell me their broadband. He asked what speeds I was currently getting and when I told him around 6 mbps he immediately told me my line would support 8mbps.

We then had a futile and ultimately acrimonious discussion about whether I would get download speeds of 8mbps from BT.

Am I right in thinking that I could not possibly get a download speed as high as 8mbps on a 20CN ADSL line?

If your ADSL modems syncs at the maximum 20CN speed of 8128Kb/s allowing for overheads the maximum speed you can actually transfer data at is about 7.15Mb/s.

8Mb sync yes throughput no 7150Kbits is the max and if your line syncs at 6Mb it syncs at 6Mb unless BT are offering to lay you a new cable and/or bend the laws of physics to get you 8Mb should you sign with them (get that in writing if they do lol)