IMA Motorsport in Virginia claims to have developed a well designed twin turbo system, albeit a very expensive one, for 987 platform cars . I spoke with them about a year regarding a 986 application (more out of curiosity than anything). It was an interesting conversation to say the least. Here's a link to an ongoing thread about their system on
6Speed :
Twin turbo kit for cayman coming soon - 6speedonline.com Forums