As an owner of a 911 I would say they are able to put their power down quicker and more effectively than the mid-engined cars, however the mid-engined cars should have an advantage through tight and twisty sections.
At the global media launch for the 911T they had some GT3 Tourings for company, through the tight and twisty mountain passes the GT3s could not pull away from the Ts but if the course was more free flowing I think it would be different. At a recent track day I did the GT cars were hitting around 230 km/h at the end of the pit straight where I was hitting around 211 km/h.