For fixing the poor performance qasdfdsaq has probably hit the nail on the head but of course the ADSL option for many is even worse.
Regarding the power level then unless it's a networks issue which would have an associated fault reference then it would normally require an engineer visit - standard fix is to move you to a lower attenuation tap in the cabinet and fit a forward path attenuator to your modem cable.
At the low downstream power levels you have they could just change the tap without fitting an attenuator. I'd hazard a guess that some engineer moved your connection when installing another - that happened to me so I had to have an engineer visit to fix it. My betting is that the agent just fobbed you off and nothing will get done - report the problem on the community forum and wait a week for a proper tech to pick it up.
http://community.virginmedia.com/