Pre July 2006 build Z4s have a known steering issue (I'll save you the details but different from your problem). My June 2006 build Z4 had the problem and in Summer 2011 and about 40K miles, BMW split the cost with me to fix it. So, while not apples to apples, just a data point for you.
2009 335i e92
2013 Porsche Boxster S