Do I expect a 5 MPG increase for the new GX? No. But there has to be some increase, especially for them to help market the new engine. If there isn't, then what is the purpose? To game the dumb federal regulations? What I do know is that Docyota was able to get that MPG on a trip he took. It just shows its possible. Hell, I have averaged 22 MPG on a couple of long highway trips in my 5.0 V8 F150.
From what I understand from the Tundra hybrid drivetrain, it's not necessarily marketed as an efficiency adder. It's to help maintain whatever efficiency it can while adding more power and drivability. The new drivetrain is more powerful, has better lower end torque (due to electric motors) and still manages a +1 MPG increase. A lot of that can be attributed to the non-ancient transmission, but it's supposed to be a better package overall (even without a V-8).