I'm assuming you mean for the 1st gen, right? Let's set aside the sheer difficulties in keeping all four TBs sycnronized and look at everything else.
PROS:
You would definitly be able to flow more air, and on a turbo that can help out a lot.
More even air flow to each cylinder.
More tunability for each cylinder since you know have individual intakes to adjust.
Should greatly improve top end performance.
CONS:
Have to rebuild the intake completely.
It'll take up a significant amount of engine bay space.
Getting each cylinder ezqactly tuned with the others will be a pain.
Outlet of the turbo will have to be fabricated to provide even air flow to each runner.
Loss of low end torque most likely.(This is an educated guess).
This is by no means a comprehensive list. But if done right and if you have the time and money you should see some good gains. Just be prepared for a lot of tuning time and fiddling with it to get every thing adjusted correctly. If it's for a street car I don't really see the use, but for the track or strip it could be useful.