If Joe Punter buys a ticket on Virgin/Qantas/Jetstar then he knows these airlines meet the IOSA standards of operation and are audited on a regular basis, unlike Cobham.
Personally I'd be surprised if Joe Punter knew what an IOSA audit was or if his airline of choice on the day was IOSA compliant or not.
Aside from that I note that Air North, Alliance, Network, Skytrans, Skywest (Virgin Regional) and Tiger just to name a few off the top of my head are not listed as being IOSA audited. Does this concern you that these companies are not applying to be audited by IATA or is Cobham the only one that worries you due to their contract with Qantas?
And as Capt Claret said maybe Easterns/Sunstate come Qantaslink fall under the Qantas IOSA audit banner and this may also extend to the Cobham 717 operation aswell. However I do not have access to nor do I feel like applying for access to the Qantas IOSA report so I don't know if it does or not.