You can find an exhaust leak by looking for soot. You don't have the lung capacity to find an exhaust leak by blowing. Or maybe you can exhale 500 cfm long enough to find a leak

If the turbo is shot you repair or replace it. You're a long way from that decision given the more likely case that the ECU is holding back performance.

The original turbo is properly sized for the fuel system. There's no point in upgrading the turbo until you increase fuel delivery. Fuel delivery isn't controlled by the ECU except at idle, and you don't want to defeat that. If you have to upgrade the turbo, upgrade to VNT rather than a larger turbo.

Please apply vacuum directly from the engine vacuum pump to the turbo wastegate actuator and see if performance improves, BUT KEEP THE ENGINE BELOW 4000 RPM. Then attach a boost gauge to the line between the intake manifold and injection pump and let us know what pressure is indicated under hard acceleration. Until you do these simple tests, you're throwing solutions at a problem you haven't defined.

Sixto
87 300D