The correct length of the OEM cable is 655mm or 25 3/4 in long, and it is part number 986 561 921 00. So, the part on that site is too long, and it cannot be installed if both eyelets are already crimped on.
If you are certain that your cables are stretched, you could shorten them if you can source a proper crimp-on eyelet as IIRC Porsche does not sell the eyelets separately.
Before you play with the cables, try a couple of the possible solutions described here:
http://sites.google.com/site/mikefocke2/convertibletopedgedoesnotfallintoguidech
Regards, Maurice.