Jon:You guys are talking about 4/3's - surely if we're talking about a warm suit you should be looking at a 5/4. My Ion Strike 5/4 often gets used even in summer.
I checked my suit and it is a 5/4/3 combination, 5 across the chest and back, 4 arms and legs, and 3 in the intermediate panels, I think, to improve flex.
Was out today for about 3 hours and no hassles with the cold, and to put things in prespective, it snowed on me whilst I was launching! 22 knot baltic winds brought some late snow and freezing temps too. (ambient 3 deg C with -2 after the wind chill if the weatherman is to be believed)
I would say a 5mm would be way too warm for Cape town. Kiters (hopefully) dont spend much time IN the water, mostly ON so wind protection is possibly a more NB factor in a suit. IMHO a 4/3 combo with decent chest panel and sealed seams would be ideal, booties would be helpful as your feet are closer to the water (unless, like hadlow et al, you spend half your session inverted).
I reckon a 4/3 with booties, and possibly a hoodie.