Compare the Jaguar XJ R 4.2 V8 Supercharged - [2003] vs Porsche 911 Carrera 4 991 - [2012]. Find out which car is the fastest, most powerful, lightest or has the biggest engine in our top trumps style comparison page. Find out which car has the highest top speed or is the fastest accelerating, is it the Jaguar XJ R 4.2 V8 Supercharged or the Porsche 911 Carrera 4 991. We'll leave it up to you to decide which is your favourite.