how tunable is the 400 diesel?

The same as any old school diesel.

Turn up the boost, increase the fueling on the pump to match and voila you've got extra power.

You could probably add more boost without having to touch the stock injector pump but i don't know how much head room you've got.
Diesels are a dark art to me in performance terms, and the turbo on these is very small to start with, so not sure at what point it passes its maximum effeciency.

Look up Renault 19 TD's engines and see if anyone tuned those as this is exactly the same engine.