You will find that 8 MBPS is a theoretical maximum, and 3.5 - 4.5 is probably around about all you will get as actual throughput, depending on contention. Changing the modem probably won't make any difference.
You could ask them to get BT to check the line, and follow basic fault finding processes at you end such as plugging directly into the master socket, however more often than not this makes no difference unless there is an actual fault.
Anyway, don't be greedy - some of us only get half of that !! ;)