I agree, I wouldn't go 3rd Party for my Boxster. I've gone through most of them recently while investigating an extended warranty for my 9-5. First of all I doubt they'd be all that much cheaper when you type in Porsche and some insist on you picking up 10% of the tab anyway. Secondly, the most popular insist on the work been done at one of their generic repair centres, which would not be of much use to you, although with some you can opt for a franchised dealer when taking out the policy, but it puts the price up substantially. Thirdly, most won't cover a potential problem, until whatever it is actually blows up. That would possibly exclude the RMS until it causes some other major failure.
Having said that if you wanted to sunstanially reduce your overall risk for a reasonable amount, you could do what I've done for my Saab, where I've taken a fairly comprehensive £2500 limit per claim policy that allows me to choose where to repair the car. I got it from Warranty Works, 3 yrs cover for £550, but I'm not sure if they'll cover a Porsche.
It would be interesting to hear from any folks who have claimed on the extended Porsche warranty. Perhaps at the end of the day it's not much more of a safety net than a 3rd party policy. I am assuming that because of the price, and the fact it's from Porsche, it will cover everything, but wear and tear, with no hassles. Any opinions?