But you are correct regarding Newlands, tartare. I think the clearance is already tight on the G/S for 16 as it is.
That being said, take-off is generally more critical that landing, so the extension may be only for 16 departures & the landing distance will remain the same on that runway, with a displaced threshold.
The reverse would be true for 34 departures & arrivals, with the extra length being available for landings, but not much good for departures. Unless, of course, you have an alternate EFATO procedure available, such as an LNAV or visual turn back down the harbour, like burty referred to.