First, keep in mind supercharged engines from FIAT are..... not just "delicated", but "particular"; I mean you should use particular care.
16v turbo is the same engine of Delta, I don't know in your country, but here (Italy) you'll find a lot of tuners which can get out a lot of hp from this engine.
You also need to "feel" the turbo: in 16v you'll feel the "kick in ass" when turbine starts.
20v turbo is the most powerfull engine (original) and you'll feel the engine as a without-turbo one (power is linear).
If you need to tune this engine you should choose very carefully the tuner because not all tuners know this engine.
But with fews changes you can reach up to 250 hp in safe mode.
The only "problem" of this engine are the belts: you have to check them carefully.
Both of engines has got the same problems: you have to use the turbine carefully (it means you have to wait the water and oli temperature are ok before push hard, wait the turbine keep cool before switch-off the engine).
Then you should check interiors (there are some differences between the t16 and t20 versions) as your personally taste.