Jamie, I assumed the rubber in the rubber donuts helped to absorb/dampen the vibrations? I would guess that metal replacements would exacerbate any vibrations? Might be ok for racing, but might be a bit too harsh for every day road use? I could be wrong of course.
that's right, there's three donuts along the prop shaft (which is alway spinning at engine speed I believe) - by breaking the shaft into two pieces, and using do nuts at the front, middle and end, in theory, you can dampen out the vibration. Until one of the donuts goes bang... So, the donuts take quite a hammering, which is why they evenutally disintegrate, although they are more or less considered a wear and tear item on transaxle alfas.