The 4.6L is better at effiecency than your giving it credit. The 4.6L "modular" was an engine based on swapping parts with other "modular" engines in the Ford industry. Therefore creating effeicency.
The 4.6L from a performance standpoint is more effecient as well giving off more power and better gas mileage. That is comparing a 4.6L that had been in production cars
only since 1996 (in mustangs)and comparing it to a 5.0L in it's prime (1995) in the mustangs. So were comparing an engine from 1999 (been in the stang for 3 years) comparing to the 5.0L (302) that has been in the stangs since the 60's.
Sounds like the sohc (an dohc) 4.6L (16v or 32v) are pretty effecient to me