I thought it was only available on PN business packages - not domestic ones.
Using Annex M does directly affect the download as it pinches part of the spectrum used normally for the download and moves it into the upload.
As it is the highly loaded part of the download spectrum it pinches, the affect on the download speed might be quite signficant.
I recall seeing somewhere that it is only really suitable for lines currently on annex A with a sync speed of >17K which have enough 'slack' to be able to compensate (to a degree anyway) for the loss of download capacity/speed.
