I think telkom sets the limit at 6dB due to aging infrastructure and damaged lines, they don't want to replace since it would be easier just to limit you.
The 6dB limit I think gives a decent amount of room for variation due to rain or any sort of interference. I see with my line that it will sync to any speed as long as the speed does not push the SNR to lower than 6dB. So by default I get 11.9Mbps sync with a 6dB limit but 14.15Mbps sync with a 3.5dB SNR. So that is a small gain but makes a difference when downloading.
I think my line is the best you can can. It's stable with little to none variation in SNR so I can max is which is very impressive for my distance. People told me I would not even be able to get 10Mbps and yet here I am on 14Mbps ;-)

