AC would do better going into CPT rather than JNB. Mornings at JNB are a zoo as three or four A380s, a half dozen 777s, and another half dozen 747s and a bunch of A330/340s all arrive between 5a and 9a! Immigration lines are epic. Compare with arriving at CPT where there are three or four wide bodies arriving between the same hours, I just strolled through immigration! Onward connections are easy to JNB and a few provincial towns.
Of course one of the problems with flights to SAfrica is that the planes sit on the ground all day losing money! Never know why someone from Europe doesn't fly a daytime flight down there, depart at 8a and arrive around 9p...then depart before midnight to get back to Europe by noon next day.
If AC had rights to CPT or even JNB via a European city (with 5th freedom rights) it could do a regular early evening departure from YYZ, get into LHR or FRA or MUC at dawn, provision the plane and depart by mid-morning. Return flight could be a mid-to-late afternoon flight back across the Atlantic giving high utilization for a 788. This is much like NZ flies to LHR via LAX, giving passengers a land break for a couple of hours in the mid-point terminal.