I haven't cheered GEM's decision to use one of the top tier...

I haven't cheered GEM's decision to use one of the top tier auditors because, frankly, the big four tier 1 firms don't seem to perform any better than mid-size (tier 2 or even tier 3) audit firms. The small firms likely lack the necessary skill set for ASX 200 firms, but the mid-tier firms may be just as good as the top tier.

In fact, a 2013 report by the PCAOB (Public Company Accounting Oversight Board -- overseer of US public audit firms) concluded: "we have no evidence of any correlation between the size of a [audit] firm and its ability to perform an audit that complies with PCAOB standards."
http://www.accountingweb.com/practice/practice-excellence/pcaob-gives-audit-firms-a-mixed-review

Meanwhile, it takes seconds to find google results of major audit failures among the top four auditors. Islay mentioned one -- E&Y's audited ABC Learning. Here's what The Economist recently reported about the top-tier accounting firms:
http://www.economist.com/news/brief...ll-cant-stop-managers-cooking-books-time-some