Poor MPG tried everything - need advice

[2bz]

Ok, my kid just got a 2008 3 door hatchback with 102K miles. I bought it for his older brother, four years ago and he handed it down recently. I am a maintenance fanatic and do everything old-school. Changed every fluid, filter (except the in-tank fuel filter), plugs, brakes, rotors, new tires, bulbs, etc., etc. Recently, I filled up the tank and noticed that it got 24.6 MPG. I asked the previous owner, my oldest boy, what he was getting and he said "never more than 300 miles from full to dead empty". Don't you just love hearing about a 4 year old problem?!?

So now I'm on a mission. So far, I have/am:

Checked tire pressure
Full alignment
Checked for dragging brakes (IR pyrometer, free rolling)
Cleaned MAF and throttle body
Not running with AC on
Using local/no-name fuel
Not throwing any codes (per local parts store OBD2)
Not heavy footed

I believe that the injectors and both O2 sensors are original issue and I just ordered reman OEM Denso injectors and new Denso upstream and downstream sensors.

What advice do you have and what might I be missing? Thanks in advance.