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.