Just done some more research - you are quite correct... This table gives the max at 172k - http://www.tdc.co.uk/gprs/table2.htm.Jip,
But VC limits you to x number of slots. I'll post the link where i saw the 43k limit as soon as i find it.
Here we can also see that the 43k relates to 2 timeslots on CS4.
I see that the max speed that you can get is also related to the multislot class that the operator provides/supports. My particular modem supports up to Class 10 (4+2) which allows for a maximum of 4 slots down and 2 up - this would give a max speed of about 86k - more on classes here - http://www.3g-generation.com/gprs_and_edge.htm
I will be interested to see the above reference though - wasn't aware of this. The last vodacom guys I spoke to referred to class 12 (4+4) if I recall correctly. Perhaps vodacom3g can comment?