"e.g keyword stuffing etc (if you go by all the webmaster tools guidelines and white hat stuff that is) yet don't appear to be penalised either which makes it hard to justify to the client why we are not 'copying', for want of a better phrase, the method used by higher ranking competitors."
Personal Opinion
Google is not the omnipotent source of knowledge it portrays itself as - It is a large corporation with communication, process, procedure issues just like any other large corporation.
But at all times, it must portray a sense of "omipotence" or constant understanding of its product...
The problem lies in the scope of the products "resources" (the inter webz)
So it relies on some Mcartyisms and Propaganda to get webmasters to "do the right thing" or even rat eachother out for their own gain, thus giving Google more easy intelligence.
So in response to NeilD i would say the reason for your clients or the question you let them answer in respect to this is "How Much Risk, and How do we Mitigate that risk, and how do we quantify it against ROI" If these questions are not easy for you or the client to answer, I would suggest running a very clean campaign. But if you have a set of disaster recovery plans in place, and the client is fully aware of what you are doing - then it is a gamble (hell half of life is a risk gamble)
Basically in short, the webmaster guidelines are just that GUIDELINES, one must make their own decisions on how to follow them.... as strict rules, or guidelines...
But the key to remember is Google is the gatekeep, and gets to change the rules at anytime, so this is the reason I personally follow them as Strict Guidelines, with a tiny bit of wiggle room is some instances.
In response to the initial question - The SEOMOZ authority rankings are based in data sets that are thousands (probably more) less intensive than the real algorithm - So the metrics you are referring to are merely a simple guideline to beginning a competitive analysis not a "source of truth"
Hope this helps!