240 Kph Limit?
 

Rented a Porsche Panamera from Avis the other day in Germany. There was a sticker in the car that said "do not exceed 240 Kph". The Panamera can do that easily on the autobahn. Just curious - what would have happened if I had exceeded it? Would I get a nastygram from Avis? Would I get punished somehow?

The car most likely is fitted with winter tires. Owners often opt to not purchase tires rated for high speeds (should be Y index for the summer tires but they went for V (thus 240km/h) for the winter tires).
In a nutshell your tires are only rates for speeds up to 240km/h. I doubt that Avis would check the various data sources to verify if you exceeded the speed limit but it's a matter of save driving.
I've exceeded 240km/h on my winter tires but with higher wear on dry roads and safety concern I just stay under.

I really don't want to sound like a schoolteacher, and this is not directed at the OP specifically, but these high-speed autobahn threads really scare me.

If you are not accustomed to driving at speed (by which I mean in excess of 150 km/h), you should really either take performance driving lessons, or just not do it. It's one thing trundling along at 135 km/h and something entirely different once you get above 180 km/h. The Autobahn is unrestricted for much of its length, and you'll be able to find plenty of stretches where you can really let it rip, but remember that it needn't be you that starts the chain reaction that causes an accident.

My greatest fear when driving at speed (and, for full disclosure, I do, but I've had plenty of experience) is someone pulling out of the centre or right lane into the left lane without looking or judging how fast I am going. When you drive very fast you need to have all your senses on and it gets very tiring very quickly. You're constantly checking your rearview mirror to see if there's a 911 RS on your tail, and always judging the cars on your right ('will they pull out, won't they?').

If you're renting, you're also in a car that you're unfamiliar with. How will it react under limiting circumstances? How far can I push the brakes before they lock or heat up? I've had my Mercedes for a few years and know exactly how it reacts. I recently added a Land Rover to the 'fleet' and am still getting accustomed to its quirks on the highways.

Bad drivers are indeed a problem on the Autobahn. I grew up close to Denmark and during the holiday seasons they flood the Autobahn because any traveler going south had to come through one choke point. Speeding is very expensive in Denmark and thus nobody would really exceed the limit by much. This means that these fellows might do exactly what LondonElite wrote: pull out of the right or middle lane at 120km/h while you are doing 220+ km/h. The difference is more than what the legal speed limit in most of the US is and drivers need to be aware.
I had to slam the brakes once in a rental car and it pulled towards the barrier - quite scary. I confronted the rental car company upon return and a week later I got a letter 'thanks for letting us know. The wheel-whatever-break-something was broken and we should not have rented the car out.'

Also remember that even a 1 series BMW (unless it's the bare bones model) has enough power to do 220km/h so there's no need to get a ridiculously overpowered car at a high price (and associated risk).

I live in a country filled with terrible drivers so coming to Germany is always a pleasure. I have lived in and driven on the highways in Europe for years and while I have gotten an Opel Corsa up to relatively high speeds on the autobahn, believe me when I tell you that the Panamera is on a different level than any other car I have ever rented including the Macan. It's really the perfect autobahn high speed cruiser.