Well, the motors Ford are now using all have Very long strokes. The 4.0 is close to 100mm, the 5.4 exceeds that I believe. The piston speeds at 6000rpm will be phenomenal, the strength of the rods needed to hold the pistons to the crank if you were to exceed that on a regular basis would approach uneconomical for a production car.
But, i've also wondered why there is a redline a few hundred rpm short of the cutout. Maybe its there to remind the driver that it isn't safe to buzz the motor that hard. But then again, I'd be assuming Ford over-engineered the I6 (and quite possibly the V8) so it'd withstand a regular thrashing, thereby negating the need for a redline - just a cutout so you dont get a hole in your hood about the size of 3.61 inches, lol!