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.
