I didn't want to jump with this in until others answered the OP's question...so all 2.5L engines have knock sensors and will adjust the timing accordingly? That means that the turbo is not the only one that yields more torque and HP at the higher revs, as you stated.
I, too, have noticed slightly improved mileage using 93 octane, based on limited [non-dynamometer] data in my Reserve. Others have here reported the same. It's interesting to see someone in an NA observe this, and at about twice the difference I've seen.
A quick calculation shows that for the OP's reported 26 MPG for regular and 30 MPG for premium, and a 70¢ per gallon price difference at today's prices in my area, there's a $3.32 cost difference for 300 miles worth of gas at each respective MPG (about 3/4 tank). The OP might find this to be acceptable for the enhanced performance he sees.