Sorry - haven't been around much lately.
This is the way I think of it. I like to think of GROSS of tax, as usually these decisions impact at my marginal tax rate, not my average tax rate..
I can get a risk free rate of about 1%pa in a HIOSA. This equates to 1% gross of tax.
Let's say I can get 8% from shares on average, being 4% average CG and 4% Dividend Yield. Let's say that just under 60% of my shares are fully franked (it's probably a bit more, but then I get nice round numbers) and I'm not selling my shares. Therefore, my gross of tax return on my shares might be 9% (4% CG, 4% Yld, 1% Franking Credit). Arguably it's a bit better than that because I'm delaying paying tax on that CG component, but that bit remains the same regardless in both scenarios.
So let's say there's an 8% average uplift in return over the risk free rate for which I might accept 20% volatility.
If instead, I have a mortgage on my PPOR with an offset account and my mortgage rate is 3% and I'm in the top tax bracket, then any $ I squirrel into my offset account offsets at 3% after tax, or roughly 6% gross of tax. Like the HIOSA, it's effectively risk free.
I could instead choose to invest that $ in shares. The volatility of the shares is still 20%, but now I'm only getting 3% average uplift in return over the gross risk free rate.
On average, I'd still be ahead investing in shares, but I'm shaving a lot off my incremental return for no reduction in volatility. And with that much reduction in incremental return, there's a lot more scenarios where I'm behind than ahead.