Does the 2004 engine perform slightly better than the 2003?

[Zing]

I just came back from a 500 mile round trip drive through Pennsylvania and I got 16.3 MPG on the highway and about 14 MPG over all. Not only am I getting good gas mileage but I think I feel a difference in my 2004 vs. a friend's 2003 with respect to torque and climbing hills. I heard that GM tweaked the engine in 2004 and think this is the case particularly given my experience on the gas mileage.