This is basically the end of the line for the naturally aspirated Porsche flat-6 as there will not be much more displacement to gain. At least Porsche is sending it off in ultimate form.
4.2 liters is indeed possible as Champion Motorsport showed us with a bored and stroked 997 GT3 RS that made 485 wheel horsepower at 4.2 liters of displacement. That was with the previous port injected 'Mezger' architecture however.
We'll see what Porsche squeezes out of their naturally aspirated flat-6 for a final hurrah.