I think everyone has covered the main items. Only other thing i can think of is the valves. If its been 50k miles since they last got checked, id go get them adjusted. :c:
My 24v with 315's gets anywhere from 20-23 mpgs. Biggest things that helped me were 4in turbo back exhaust, I know you already have one, Afe air intake, DDP 50hps injectors, a BD exhaust manifold, and an Edge programmer. Getting my valves adjusted helped HUGE! :tu: