I flew both 260 and 300hp Islanders into the Kabwum Valley to and from Lae airport. I don’t recall any issues when it came to climbing. Obviously the 300hp was better, but most of our Islanders were 260s. Maybe the locals were skinnier back then?
That said, we used to fly east/northeast out of Lae to avoid the Nadzab CTA steps, so we went around the big rocks rather than over them!