Impossible to tell a direct difference in MPG, but .4L less may be good for a couple mpg. Too many variables.
If hes looking for mpg, and it has to be subaru, and has to be awd, why an auto?
Ive managed 34mpg on a 55mph highway trip according to my ultragauge, but I'm sure I could have done better with a 5 speed. An EJ18, 5 speed fwd could probably manage 40mpg if driven right. Especially in a coupe.