It's very difficult to manage these kinds of long term issues. But Telkom do have specialised units with reports on throughput and the quality of your connection once they monitor it. Hopefully that's something that can become available if their general technicians are not able to assist.
With regard to compensation, you can log a dispute with Telkom, and in some cases they do credit you. Generally this is in cases of no service at all for an extended period. They may regard a working (though slow) connection as within the best effort service provision. I guess it wouldn't hurt to try though