I suspect Comair's operational base at JNB may be more relevant.
Regarding the 737-800's lesser capability, is this not only at MTOW; with a finite load, say 100 pax, I understand it performs as well if not better than a 737-700.
As I understand it, bulk fuel will not be available at St Helena, and thus the aircraft needs to carry round trip fuel when departing South Africa, which is what makes the JNB departure (at 6,000 ft elevation) more surprising compared to sea-level Cape Town. They would need to carry that anyway because there is no diversion point short of return to base.