It was my impression that vehicles aren't supposed to be able to go faster than the speed rating of their tires. Since 250 km/h is a common speed rating for tires, and higher ratings tend to get expensive fast, German cars are often limited to 250 km/h.
No current Porsches (real ones, not the SUVs) are that slow, and the Porsche tire recommendation guide calls for tires rated for 300 km/h, or "over 300 km/h" (the standardized ratings don't go higher) for all models except the Macan SUV with 18" wheels, for which it recommends 270 km/h.
http://files2.porsche.com/filestore/download/usa/none/porsch...
You can thus see why the index of stock tires always vastly overshoot the top speed of a car, e.g my car gets a Y tyre stock even though its top speed is 245kph, and my ex car has a W rating when the top speed is 210kph.
It's not a good idea to go anywhere near the limit of your tire speed index, nor to go cheap and skimp on the index when buying a replacement tire.
I bet the Macan that gets 18" and the W index is the one with low enough top speeds (below 230kph, that'd be 2.0L four-pot petrol and 3.0L V6 Diesel ones)
EDIT: found a reference again WRT index tests
> Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=...
Anecdote: recently went from Offenburg to Stuttgart and back and could drive a pretty much constant 190~240kph all the way (~1h30) except for a couple sections which were not unlimited. Having to slow down to 190kph isn't quite what I'd call a traffic jam. 300kph is just harder because there's not that many vehicles able to go that fast, so you end up catching up with the "slower" (so to speak) ones, and since vehicles are (supposed to) be electronically limited to 250kph, you're bound to meet some anyway.