The HG612 supports Annex M. If you use DSLstats there's an option on the Login configuration page "Connection is Annex M", so you don't need to issue any commands manually to monitor the stats.
Be aware that Annex M isn't available everywhere, and not every ISP will offer it, so it's a lottery as to whether you will be able to get it.
The effect on downstream speed is roughly 1:1, i.e. the loss of downstream speed is roughly the same as the gain in upstream.