Investment property & diversifying

I have been thinking of late about a new investment strategy for my very first house i brought when i was 18. I am almost 10 years into my mortgage with another 15 to go; i pay principle & interest as i have always done over a 25 year period (approx 15 years left).

What i am thinking of doing is putting on a loan variation to extend the mortgage over a 30 year period and convert payments to interest only for 5 years. This would allow approx $600 per month for further investment.

What i was thinking is diverting that $600 to an Australian Shares Fund (probably perpetual that has returned with longterm returns of approx 12%pa after fees).

Or

I could own the investment property in 15 years time outright.

Figures i worked out:

Investing:

$300 per-fortnight with 12% average return time 30Years = $2.4Million Approx

Issue: still in debit at the end of term & $881,400.00 interest payment (tax deductible)

Owning Sooner:

* Nice passive income from rental property in 15 years
* Minimal Capital Growth (coastal property)
* Debit free
* No tax offset
* reduced Diversification

Other notes:

I have TIME with my age and can afford to sit out short term market rides

Just wanted thoughts from people or feedback from people whom may of done this?? and if this was a good strategy for you?? or if i am completely being silly with this thought?

Cheers

Jeffery