Hum, they are at 126HP per L... pretty stout. That is almost 600 in a 4.6L V8. The almighty 2013 Shelby has less HP per L at 120.4. My 04 Cobra is at the limits without major $ being spent at 157HP per L (but it did take a lot of $ to get there).
So with all that said I sort of wonder about the capability of the all aluminum block to go much further... well I am positive Ford is a good conservative % below the capability of the motor for longevity. It will be interesting when someone finds the limit. But I do agree with Sleepin 4.6 people will find 300 when they get the tuning down pat.
2013 ST2 for DD
2004 Terminator for fun