I did it about a year ago and doubled my (somewhat slow) broadband speed.
BT say my line supports 256k to 512k, however I get just below 2m (out in the country you know).
As long as you have a micro filter on each extension, you do not require a bell wire because the filter has the capacitor required to generate the ringing signal (previously only in the main BT line box).
Many other countries use only a pair of wires but the legacy Post Office system and 'steam powered' telephones in the UK required the use of a bell wire.
Give it a go, what have you got to loose. Just remember to disconnect the bell wire at both ends, i.e. the line box and the extension box.
JsJ.