There are a few cams available that will increase your top end, but typically at the expense of your bottom end. The other consideration is the added lift and duration will need to be addressed. You will need to make sure that you will not have any piston to valve problems or problems with coil bind. If you go to
Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles you will find a few choices.