"Is there a performance issue with relocating the cat to the underside of the car nearer where the n/a cat would be?"

Yes, I think so.

As you know, the cat must run very hot to be effective. It gets its heat from the exhaust gas.

As you also may realize, the turbine extracts energy from the exhaust gas by letting it expand -- and in doing this, the exhaust gas cools (this is thermodynamics 101). The further away from the turbo it's placed the cooler it runs. And if the cat's under the car, it is cooled even more by the cold air. But if it's directly behind the turbo it benefits from the hotest exhaust and is also shielded from the cold air.

The B230 turbo has a greater volume of exhaust than a B21, and the turbo may actually extract a bit less energy -- and thus cool the exhaust less. The B230 turbo receives enough heat even if it's placed several feet away from the turbo.

I don't know if I'm right, but it sounds good.

'83 is getting on in years -- does your state still require an emissions test for it?