driving habits will do more than any mod you can do to the car. Common sense...starts and stops...slowly, conservatively. Keep tire pressure up, but with tpms, thats easy. Maybe a K&N filter. But really, the way you drive will do way more than any "mod".
FWIW, our 09 IS 250 AWD consistently gets 27 to 29, with more on a trip...they really can be very economical cars!!!