I get about the same...
I've been getting about the same speeds on GPRS with a 6680 on bluetooth at my home. Peaks at about 5.5Kbyte/sec, which is 44Kbps. Never measured the uplink speed -- I'm just watching the traffic graph in KPPP.
But I thought that GPRS+ was available everywhere on MTN. Doesn't this mean that the phone can switch to CS3 & CS4 in good signal conditions (GPRS only uses CS1/CS2?), which should give a maximum bitrate/timeslot of about 20-23Kbps? I'm about 800m from the cell tower and typically have 6/7 bars or full signal strength, but I don't think I see speeds over 40-50Kbps.
Also, according to the coverage map, I should have EDGE coverage at home (Cell ID is Mtunzini, I haven't seen Y'ello Broadband). I'm wondering if this is still in the pipeline? Strangely enough, I don't yet seem to have used EDGE -- I'm either in a GPRS only cell (Mtunzini, Strand, Empangeni/Sanlam Center), in a 3G cell (DBN & CT airports). 3G gave really good transfer rate peaks -- 200kbps->400kbps.