Ultimately, the limit of a diesel tuning box is the factory software because, the tuning box does not change the factory software.
We manipulate the fuel rail pressure to get the engine to inject more fuel but as engines get more advanced, they will use more sensors to monitor the engine.
For example, early turbo diesel engines did not have EGT (Exhaust Gas Temperature) sensors or wideband lambda sensors so there was much more freedom to turn up fuel pressure.
Modern engines can monitor EGTs to double check the amount of fuel that is actually going into the engine matches what it thinks is going into the engine.
So in short the limit of a tuning box is the operating window put into the cars software from the factory. Some manufacturers give a lot of room to increase power, others not so much.
Having said all that VWs diesel gate will ensure that modern engines will be sand bagged from the factory for emissions tests.
ie an engine will be capable of making much more power than it does from the factory because it has had to be detuned to meet emissions targets.
Now and in the future, tuning boxes are the easiest way to realise the full potential of the engine ie to release the potential in the engine but which car manufacturers have had to hide for emissions reasons.
How does this tie in to the limits of a diesel tuning box?
It means that you can expect the power gains from tuning boxes to increase as emissions targets for car manufacturers become stricter.