Hi rehder,
What dates are you going to be here? I'm based close (+/- 1 hour drive) to Cape Town.
Although it's a hit or miss affair, at this moment there is fairly good Yellowtail spearing at the three main areas targeted by spearos from Cape Town:
Dassen Island on the West Coast, which is predominantly a winter spearfishing spot, has been producing sporadically over the last few weeks - we got a few last weekend but it was pretty hard work with the fish being very very skittish.
I got some reports during the week that quite a bit of Yellowtail has been moving into False Bay, which is traditionally more active in summer. This means that they will sooner or later be targetable at areas such as Cape Point, Vishoek, Glencairn or Rocky bank.
I also got some reports that some Yellowtail had shown up on another predominantly summer area, the Struisbaai banks. This area usually holds the bigger class fish and it's not uncommon to hear of 15kg+ fish being speared here.
Although Cape Point offers some shore diving options, when targeting Yellowtail, all three areas mentioned are really only accessible by boat.
The Tuna season seems to be running late, usually by this time of year some boats would have landed some but it's still pretty quiet. The warm water is still too far away (60-80 Nm from Cape Point) for smaller ski-boats to travel out there, but this should change sooner or later with the South Easterly wind which pushes the warm water closer to land already having blown for a couple of days.
Although the best diving in and around the Western Cape is usually done by boat, there are some good options available if you want to shore dive. Cape Point offers some pretty decent shore based spearsfishing with reef fish on offer at most of the spots. Due to the shark factor and marine reserves we usually do not shore dive any areas in False Bay from Vishoek all the way around to just beyond Gordons Bay. Beyond that there are also some good areas to shore dive, the link in your post discusses some of theses areas. Driving up to areas such as Struisbaai, Arniston or Infanta, which are all in the 2 to 3 hour travel time range from Cape Town, could on the right day yield some phenomenal shore based diving.
Some red tape: You will require a spearfishing permit which is available to buy from most post offices. Also take note of marine reserves and catch restrictions. You may download more info with regards to this here:
http://www.daff.gov.za/daffweb3/Branches/Fisheries-Management/Marine-Resource-Management/Recreational-Fishing
Let me know when you are here then we can possibly make a plan get together for a dive.