Saw this 1st drive report, and they quote Heinz Krusche as saying you'll have to wait for the ///M version for the really stiff suspension.
http://www.thedetroitbureau.com/2009...-drive-bmw-z4/
With 5 sec 0-60, I'm not sure there is a business case to build a ///M version.
Especially if the 3.5 version is starting at $52K