Hi Cirque,
July to October months are generally not the best time of year for spearing here as the winter storms that batter the Western Cape make diving less than ideal. During these months Yellowtail can be taken on the western side of the peninsula but the areas where they are normally targeted, Robben and Dassen Islands, are only accessible via boat.
Cape Point is one of the few places in SA where Yellowtail are speared on a regular basis from shore dives and most probably your best bet for good diving close to you. Black Rocks, Bordjies drift and Rooikranz are generally the best areas in the reserve to try for Yellowtail but they can also be targeted on the sea facing edges of the big kelp beds on the south western side.
There are some spots closer to the city itself such a s Seapoint and Campsbay but shore based diving in these areas normally only yield smaller reef species such as Hottentot. On the other side of the peninsula Falsebay holds a wider variety of reef species, such a Red Roman, Galjoen, Knifejaws, Steenbras etc but tends to be more sharky and there are a few reserves in the area where no spearfishing is allowed.
In terms of gear, your 110 gun should be good for most spearing in and around Cape Town and you can always upgrade it with a roller head should you require more performance. As for a suit, it depends how susceptible you are to the cold but your surf wetsuit may end up being a bit on the cold side if you do extended dives - especially on the Western side (Atlantic) where temperatures of 10 - 11deg C are not uncommon, so it may be worth buying a chicken vest with a hood or investing in a dedicated spearfishing suit.
The best way of getting to know the dive spots and the conditions that make them work is to dive with other people that know the area. As far as I know UCT does have an underwater club so it may be worth getting in touch with them to find out if there are any spearos in the club. Alternatively get in touch with Blue Spearos Club (bluespearos@gmail.com), they are the most active club in the Western Cape.
If you are really after good spearfishing then the North Eastern side of the country, Transkei to Cape Vidal, is much better during that time of year and a trip to these areas could yield some phenomenal results.