I have an '03 G35 Coupe. Automatic.

 

I do too and have been FI'ed for almost its entire life.I first did some of the N/A mods and really wanted more so the first supercharger i installed was a mistake as it was the Stillen thru all its stages and wanted more.

The next mos was the Greddy twins and have the power i want now.BTW you guys wont get a twin turbo kit for less then $6500 (new) any more and then you have to look at the installation and tuning costs.Mine has been dead reliable with over 75,000 miles on boost.

 

So how much was the stillen?

 

your best breather mods will be:
Z-tube
Motordyne 5/16" plenum spacer
True dual catback exhaust (MotorDyne & Ark Grip are very popular and well repected).
HFCs or Test Pipes, HFC won't throw a code and don't have the in cabin exhaust fume smell.
Some people like to add the 06' revup air box as well.
The oem setup with z tube tends to have a lot less heat soak from the engine than aftermarket air intakes.

 

The Stillen stage 4 was $5700.00 but was very easy to install and dosent need a tune as it comes pre tuned with a fmu but dont waist your money on that kit...trust me on this.

 

^^Listen to him. If anyone on this board can speak from experience he can.

 

Go with FI if you have the coin to drop on your ride.

I have in place a Injen CAI, Fast Intentions HFC's, and Motordyne spacer. All have increased power/torque somewhat but nothing too significant. I'm guessing I've probably only gained 20HP (if that) with all the things I've added. Not cheap either for all of that.

FI is the way to go but expensive.