Will, I'm not really sure what is left to troubleshoot... I am well past any notions that the speeds I am seeing have anything to do with my account and/or my line. It has been an exhausting three days of investigation on my part, having worked closely with 8 friends of mine who are also MWEB subscribers, and the result of that investigation is clear. I'm really not interested in jumping through the usual troubleshooting hoops of local speed tests and logging faults with Telkom when I have long-since eliminated these things already. In fact, I am SO brazenly confident of my test results and that it is NOT in any way related to my own personal situation, that I will go so far as to issue a challenge!
I challenge ALL 4MB MWEB Uncapped users in the Durban area who have either a Dropbox account (or similar) or Web Hosting in Europe or the States, to try and download a file of 20MB or more at an average rate of more than 75KB/sec using a single connection (so no download managers). Time your download and work out what the average speed was at the end (don't look at the realtime speed). Once done, rename the file to bypass any possible local caching that has taken place, and then give the URL to a Cape Town user. I suspect you will find that the user in Cape Town will get the same file at least two to three times as fast.
From the comments here, I'm pretty sure that JHB users can do the same tests as JHB / DBN traffic all seems to go via Midrand. However, I have not personally tested the speeds in JHB so I only have the DBN / CPT comparison to go on.