Many internal parts may be interchangeable but they are not (?) drop-in replacements. Good used 5 speed transmissions are not expensive and are easy to find. 6 speed(Getrag) is a different story.
Here is a link that gives you the differences - ratios:
http://986forum.com/forums/performance-technical-chat/35080-986-five-speed-tranny-unmodified-audi-012-a.html