Power figures sell cars, torque wins races.
Bore and stroke on the 3.6-litre engine is 100mm x 76.4mm; on the 3.8 they changed it to 102.7mm x 76.4mm. On the 4.0, they kept the same piston size of 102.7mm and increased the stroke by 4mm to 80.4mm
According to Andreas Preuninger, Porsche motorsport manager for high-performance cars: "We have finally really reached the bore limit with this engine. This is the maximum before we need to create a new engine blueprint." Wall thickness between bores is in fact so thin now that steel cylinder liners have been added for durability as with the GT3 RSR racecars.
In my experience of doing a 3.9 conversion (using 104mm pistons) but retaining the standard crank & rods, going over the 102.7mm bore of the 3.8 results in the piston flopping around inside the cylinder because they have to allow for the piston to expand and the cylinder expands much less than the piston; the greater the diameter, the more the piston expands when hot and this larger cold clearance increases wear. Not to mention sounding like a bag of nails until fully up to temperature. I don't have that engine any more due to piston / ring failure after 42K miles. There's a reason that Porsche don't use a larger piston & chose a longer stroke for the 4.0 ...
AFAIK, nobody has made a N/A Mezger engine of over 4.0 produce reliable big power for hundreds of hours on a track. Claims of 50K miles on the road in big bore cars are out there but that's not really stressing the engine. Just because it's possible doesn't automatically mean it's a good idea...