Nengun has them too -- they definitely work. It pulls my oil temps down by at least 10-15* based on the OEM gauge and during dyno-run intervals, cool down rate is much faster, confirmed after dynoing on the same day with another Z in FL some time ago.
It adds metal and uses a heat conductive adhesive (not merely double sided body moding tape), so as jerry-rigged as it sounds, the laws of thermodynamics are still effectively applied.
ARC - Cool Fin - Nengun Performance
I doubt it would take more than a few weeks -- although, if you are springing for the finned dif cover, while adding more fins certainly won't hurt, they won't be critical either.
That said, you can add them at any time and it's a good bang-for-buck mod to be sure.