IOSA in my opinion is just the bare minimum to scrape by. It does set a standard, however not a very good one because that would hurt IATA membership.
We're just in the process of joining an alliance (not star though) and the audits, both on the flightline as well as in the office and back-office side, of that alliance were extremely thorough. Our regular IOSA audits didn't impress them much and the required standards in pretty much all parts of the operation were quite a bit higher.