To answer your original questions,
Porsche 911 rear seats wont fit in the slightest, they are little more than pads that sit in the rear bulkhead/transmission tunnel. Corrado seats can be fitted as nick has helpfully linked to BUT in my opinion because they are narrower they never look right. Corrado front seats are just as crap as mk2 gti seats, OK they are slightly better shaped and sit lower, but very little compares to golf recaro's for factory option awesome-ness. Cost a bit but you'll never go back!