I thought I would throw my three penneth into the discussion about BT infinity.
I switched to BT on 18 May. I am 3900 ft from the cabinet. Thats measured by line reflection by the BT engineer and is pretty close to the OS map and Sat Nav ( I know the cable route). On day 1 I got 52Mbs down and 8 up. It settled down to about 36 Dn 8 Up by the end of the day. However, the modem was dropping PPP 4-6 times an hour, but the speeds didn't drop. A week later Openreach brought me a new modem which didn't drop PPP.
When BT took over my phone I started getting noisy phone calls which I had never had in the past.
After a few weeks I bought a Draytek 2750n which worked straight away. I could see from the line stats lots of UAS errors on the upstream. The margins were about 6 db Dn and 3 db Up. Over the following weeks the Up margin climbed to 12db Dn remained at 6db but the dn speed dropped to 23Mbs and Up to 2Mbs. The electrical line length was climbing too.
After weeks of complaining and several engineer visits to give me new modems I didn't need, BT finally sent someone look at the line. He replaced the drop cable, modem and the hub. It didn't work at all then. He then got me moved onto a new pair and port in the fiber cabinet and got my line "flexed". Everything worked then. The speed came back in both directions.
He told me I wasn't allowed to use any other modem, which was the first I had heard of the condition.
Speed tests with spedtest.net and BTSpeedtester gave consistent results about 34 Dn 8 Up.
After a week I switched back to the Draytek. The down speeds are the same but the up speeds are about 3-4 Mbs on Speedtest.net and 5-6 Mbs on BtSpeedtest. I then found TBB speedtest. It gives about the same down but about 8Mbs up. Putting the Openreach modem back gets 8Mbs up with all testers.
Dsl reports tweak test shows my MTU up and down is 1442, it was 1492.
My margins are 13db Up, 6Db Dn. Sync is 9998 Kbs Up, 36350 Dn. My electrical line length has dropped and is now 19db.
BT have not been able to solve the noisy calls problem.
Draytek have sent me new firmware and are trying to figure out why their box wont achieve the same speed on the up side as the BT modem.
Bt support has been useless through out. If they hacve anybody who understands VDSL I cant get to speak to them.