Actually the technical specs from Porsche.co.uk show that the Cayenne 3.6 post a time for the manual of 8.1s 0-100km/h and 8.5s for the automatic. The BMW is 6.7s but is this for a manual or automatic.
Anyway here's the link to the Porsche website.
http://www.porsche.com/uk/models/cay...turesandspecs/