Will the cobb even fit the 03 sedan or will it only fit the coupe that year?

 

The COBB AP is garbage for the the G. Works well on Mazda and Subaru, but Nissan/Infiniti's best option is Osiris Uprev. Plus, they've discontinued support for the G, so its not the best idea. Look at page 2 of this thread and read seymore4's posts. He can etune for you, and it does wonders.

 

Yeah I think I have decided to go with osiris. What is the usual cost for an osiris tune?

 

First you need to verify that you have a later 03 sedan with a compatible ECU.. Early models have the old style and are unable to be reflashed. You have to physically look at it to verify.
https://uprev.com/documentation/Supported%20ECUs.jpg

Osiris standard with reflash cable so you can reflash yourself at home is $500, dyno tuning usually runs $600-800 with no reflash cable

 

So I have noticed that the main differences between the late 03 model and the older 03 model is that the newer one has no handle or release on the trunk itself and instead has "Infiniti" on it, a second power supply on the passenger side, and the driver seat controls are on the right side of the seat and not on the left near the door. Are those claims true? I also noticed on my ecu that it has a silver tin case and i think it has the bolt pattern to the third one. What is a safe way to remove or figure out which ecu it is?

 

Look at the ECU, and compare it to the pictures in the link I provided. It will be VERY obvious which one you have