the resonators are not going to hurt its the catalytic convertors could cause a lean condition in the engine and destroy it.

do the spark plugs (i feel you should do the easiest, cheapest, feasible things first, in the discovery of a problem.)

the car is engineered to operate OPTIMALLY for power, emissions, longevity etc. thru the current design.
thats not to say that you cant make certain changes for better performance but understand that if you do then those things listed above will change.

the term back pressure tends to get some performance types in a war of words .... OK no problem. the 1uzfe pipe SIZE allows for a better term the increase of exhaust gas velocity changes to it could cause the bad gas mileage you speak of.

the 02's give a code if they are faulty.
When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and USES MORE FUEL than it needs to.
there are pre cat. and post cat. the post cat. 02 sensor moitors the cat. with NO cat you may have a problem.
yours are located on the exhaust just under the manifold in front of the cat. and after the cat. 2 on each side for a total of 4.

 

so you're saying i cut off my cats (the o2 sensor was on it too, i dont know where it is exaclty) ?
right now i can only change spark plugs and tranny flush the rest i've already done.... will this improve mpg or not likely?

and my friend says as long as check engine light isn't on o2 sensor should be okay

 

do it and see, if it improves great.

all you can do is 'tweek' the system in this case until the problem is resolved or at least livable. inspect the various components on the car to get maximum results. check for air leaks and fix them. look for loose wires, corrosion on electrical connectors etc. all things you can do from just looking under the hood.

ex. i generally extend a tune up to checking/adjusting the accelerator linkage checking the tire pressure for optimization. hell you can even remove all the JUNK in the cars trunk back seat etc. when looking for optimal MPG.

if no check light on o2's cool... no need to fix or attempt to correct if no light.

 

No ones mentioning it so I'll throw it out there..
Make sure ur tires are properly inflated. Poorly inflated tires will give u bad gas mileage. Also, make sure you use high-grade fuel only. These cars are made for high octane fuel, they just burn through low octane fuel like crazy.
May be too obvious, but those are two REALLY simple things that'll help if ur not already doing them.

 

You're paying too much for mobil 1, but then again you are in ny. I pay around $35 bucks for Mobil 1 high mileage and it comes with a free Mobil 1 oil filter.

I had the same issue with my 96 sc300. I had low mpg and someone recommended I use Red Line SI-1 Complete Fuel System Cleaner. It's a fuel additive that cleans your injectors, carburetors, valve and combustion chamber. After that, my sc got better mpg.

 

Has the coolant temp sensor been changed yet?