still getting abnormal gas mileage..

[mastershake]

when i first got my alero, it was getting 11 mpg (mostly highway)

since then ive:

changed oxygen sensor
changed air filter
cleaned throttle body
cleaned maf sensor (got rid of oxy sensor code)
changed spark plugs to ac delco iridiums
changed spark plug wires
changed oil to mobil1 synth

now im getting about 16 mpg highway, which is still not acceptable. i do have a low cat efficiency code but there are no signs of it being clogged (loss of power, rotten egg smell)

so is it really the cat converter thats causing my bad mileage? if not what else is there to check out?

also i check tire inflation almost every week, and i store NOTHING in my car to weigh it down. so those couldnt be part of it..