So - the highest possible speed in SA on a 10mbps link is 6mbps.
Why not call the service 20mbps, or 100mpbs, or 1gbps for that matter. Call it infinite speed.
"ISP xxx now offering the fastest internet ever"
It doesn't make any difference as it seems you don't get what you sign up for anyway.
And (as ISP xxx), why not do a whole lot of speedtests at 3am from your core when usage is minimal - unthrottle the connection and really spike up the results so your company comes up on top. Customers won't know the difference - they're going to get 6mbps (or less) in any event