OK, here is my two cents. The 6.1 puts out 69.7 HP per liter and the 5.7 puts out 59.6 HP per liter. This is because the 6.1 has been tweaked with high perfromance parts, such as bigger cam, valve springs, more compression, bigger valves, better exhaust etc. In addition, it uses higher quality parts to improve reliability at higher rpms. The 5.7 is a high volume prioduction engine and uses standard parts. No sodium filed valves, forged crank, etc. Since the engines are the same basic design, the 5.7 has the potential to produce similar HP per liter numbers as the 6.1 with proper modification. Since the 5.7 is producing less power realtive to its potential, the gains from modifications will be greater if the engine is modified wisely. This does not mean that a 5.7 will produce more HP than a 6.1 given the same mods, because it won't.
The 6.1 is a premium engine and to get a 5.7 to produce power at the same level with the same reliability would require the same quality of parts.