In the 4.6 premium every forth tank, for the R-Spec, I put premium every other tank (primarily to control the bumper soot issue).
Here is what I get (using scientific means = miles/receipt GAL):
4.6:
Best MPG: 30.7, Atlanta to Miami, at 26500 Miles, using CC, speed 75-85+
AVG Daily trip - CC: 23.4 mpg.
AVG Daily trip - Light foot : 20.1 mpg.
AVG Daily trip - Lead foot : 16.1 mpg.
R-Spec:
Best MPG: 27.2, Atlanta to Orlando at 3600 Miles, using CC, speed 75-80
AVG Daily trip - CC: 21.6 mpg.
AVG Daily trip - Light foot : 17.5 mpg.
AVG Daily trip - Lead foot : 13.8 mpg (I blame that Mustang for 'following too close'!

)
AVG Daily trip = 10 miles 45 mph strait line, 9 traffic lights.
CC=Cruise Control
Hope this helps.
One other suggestion, is to keep in mind that the 3.8 has a softer suspension setup than the R-Spec so you may want to drive both to establish which one you like better.