FWIW, it's a myth that compressing insulation decreases its per unit depth R-value. It would do so if, in the act of compression, a gap or air gap is created. Standard insulation will gain (per unit depth) R value up to almost 50% compression. Cornings have a chart of compression values for their products.

If you go to BRANZ website http://www.branz.co.nz/cms_display.php?sn=167&st=1 and view the GUIDELINE for December 2012, you will see a note about insulation over specification, and the problems it can cause - Rediced R-value, difficulty in getting a tight fit to the framing, causing a bulge in the flexible wall underlay into a drained and ventilated cavity, and the popping of screws as the sheet adhesive is not properly compressed.