Agree with Scott, plus I think that a rear mounted rack works fairly well on a 4x4 or hatchback because the car is bulky and shields the bikes from the wind, reducing wind resistance. But on a 911 the car is too low/sleek to achieve that shielding. With the bikes side ways they would act like a parachute, whereas, on the roof, facing forward/backward will create less drag.
I use a pair of Mont Blanc "Discovery"s - mounted on the official Porsche cross bars. The Discovery is the updated version of the model Porsche sell under their own name. It is much stronger than your average "Thule" and has advantage of sitting the front wheel all forward of the front bar, so it works well on a short roof. I'm very pleased with them. No rattle or sway - including driving enthusiasically around many hairpins in northern Spain last summer.
If you are buying a pair note that they come in "left" and "right" models (neither my shop nor I realised this at that time I purchased) but you can, with a torx key, convert one around which means it's easy to get at the lock of both of them without reaching across roof of car!