SAA's carry on restrictions are 18lbs per bag. I have a LowePro fast pack 350(4lbs) and pelican 1510 (14 lbs)
This is what happened to me last year on a SAA flight, flying Business Class, Niarobi to JoBurg.
2 of us Flying BC, 2 Carry on Bags, a Battaflea weighing around 8Kgs, and a Carry On Wheeled Pelican, also around 8 Kgs.
At the door the Cabin Staff were taking bags indiscriminately from Passengers, tagging them and sending them to the Hold, they made no allowance for Business class or Commercial Class, they simply didn't care, and arguments to the value of the equipment fell on completely deaf ears, again, they didn't care, they were rude and quite honestly some of the worse Cabin Staff I've encountered.
In particular in my case they targeted the Pelican Case, which was tagged and disappeared, they said to the Hold.
And of course from the entire flight my Pelican Case, with around $20k in Camera gear, was one of 3 bags missing, all Pelican Cases, all containing Camera gear.
In my case I had some very good friends that were able to go to top management in SAA to complain, my Bag eventually surfaced in Nairobi, it had been tagged at the door, then removed from the flight, make your own conclusions.
I sat in JoBurg for 3 days waiting for the Bag to get to me.
My situation I've since found is not unusual, In Africa some Cabin Staff do target Camera gear, some ground staff do target camera gear, the only advice is if your inside the carry on allowance, refuse to board if they want to take your gear off you (this can make them re think as they would have to remove your bags from the hold & that will delay the flight), and ensure full Insurance Cover.
I've travelled in Africa for the past 10 Years and this is the only time I have had this sort of issue, but it has put me off SAA, of course there's not a lot of reliable airlines in Africa to choose from.
One thing to remember, if they quote your over weight by 3 or 4 Kgs, open your bag, remove a Camera & Lens, hang it around your neck, or shove lenses in your pockets to get you under, then they generally give way.