Speed will soon drop back again
It is true that after filtering noise from the bell wire, an ADSL modem will synchronise at a higher speed. The benefit can indeed be significant, claims of 1-2 Mbps are quite credible.
But here's the catch...
When a new customer in the UK gets connected to BT's adaptive 'up to 8 Mbps' service for the very first time, the line is subjected to a 10 day stabilisation period. During that period, a gadget at the BT exchange monitors the line rate, and establishes a so-called "Fault Threshold Rate" (FTR), which is the rate that your connection appears to be capable of sustaining. Thereafter, the FTR never changes unless BT decide there's a need for it, for example if they replace or update the exchange or it's wiring. Once the FTR has been set for your line, BT are meant to try their hardest to ensure you always achieve your FTR, or better.
Following the 10 day period, BT tweak a parameter known as 'target noise margin' in order to control the speed at which you connect. If the target margin's too low you'll connect at a higher speed but will experience lot's of errors. If the margin's high, you'll connect at a lower speed but your connection there will be fewer errors. Bt like to err on the side of stability, since it reduces retransmissions which could congest their network.
Day in and day out, BT's equipment constantly monitors your line to tweak the target margin, using an algorithm called 'dynamic line management' (DLM), which is based on the FTR that was established during the ten-day stabilisation period. DLM aims to make your line as stable as possible, consistent with allowing you to connect at something close to your assigned FTR.
The net effect is that if you install one of these devices your speed will increase temporarily until, some time later, DLM spots a few transient errors on the line (e.g. because of a nearby thunderstorm) and because your line is faster than the FTR, it will raise your target margin to reduce the speed back towards the FTR. This may happen within days or it may take a few months, but it is almost certain to happen. Once the target margin's increased it is very unlikely to ever drop back again, it's effectively a permanent change.
So try this by all means, but don't expect the speed benefit to last. In fairness, your line will be more stable than it was, but that's another matter. Alternatively, if you have the energy, petition BT to restart the 10 day period when one of these is fitted so you get a higher FTR. Except you can't talk to BT, you have to talk to your ISP, who won't want to know...