You should be getting over 21 Meg, but you should be running on G992.5(ADSL2+) not G992.3(ADSL2).
Can you post the rest of that stats page showing the error counts, preferably after hard rebooting the router and letting it run for at least a few hours? Together with the Time Connected: from top page.
Are you plugged into hidden test socket behind faceplate? If not, do so.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC