It's getting cold in Cape Town and my grow is averaging the same temps as yours, 15c is perfectly fine. If you start getting to 10-12c every night i'd consider a heater. It's only getting colder from here onwards so i'll probably grab one when we are in the middle of our winter. Colder temps bring out some great colours in some strains, so i love it when we get to this time of year.
If you are having humidity spikes in the evenings get a heater, it will help.
I have always flowered on a 12/12 schedule. If you'd like to speed up the ripening process go ahead and increase your dark period, but your yield may be effected.