13 years is a strange timeline. Here's my source
https://seekingalpha.com/article/415...vs-s-and-p-500
Looks like they did well around 2007 which bumps up your quoted 13 year returns. The trend is pretty clearly towards matching the market in the future, as you'd expect as the size of the company increases. Not a terrible place for your money, but don't expect the future returns to look like the past.
I can fully understand not wanting to be in kiwisaver if it's too inflexible. On a pure financial returns basis, I will take it over Berkshire anyday due to my employer and the tax man more than doubling all my contributions.