SPP Harvester by OnMarket, page-7

If there are 10 SPP's that you take part in per year & the average return is 15% that simplistically is 150% return p.a less 20% performance fees is 120% p.a.

If you invested $30,000 less $3,000 application fee, $300 brokerage fee & 1 share in the 300 companies is currently approx $3,650 that leaves you with $23,050 to invest in each SPP.
Assuming you get a full allocation that's $27,660 profit from 10 SPP investments.
Assume the cost of buying the 1 share in each company is a write off that means you have at the end of year 1.
$23,050+$27,660=$50,710, a profit of $20,710 or +69%

Year 2
Same net return of $26,660
End of year 2
$50,710 + $27,660 = $77,370, a profit of $47,370 or +158% / 79% p.a.

OK you probably won't get 100% allocation every time but either way it's a pretty compelling investment assuming you invest more than the $15k because the above doesn't take into account the compound effect.
Plus the $3,000 + $300 costs are deductible from the profits before the performance fee.