Has anyone experienced any significant gains in speed using the BT I-Plate or by disconnecting the Bell Wire ?
Yes to both. Also, ensure that your wireless router is connected directly to the primary socket (i.e. to the i-plate) and you have no other internal house wiring, not even a "proper" telephone extension cable bought in a PC shop. I took out some dodgy wiring installed by the previous owner and got a 2Mb boost after 48 hrs when the exchange equipment re-adjusted the line characteristics.