Fred's right on the money.
Replace your airfilter with a Nissan unit - Not that expensive, call Joe at Scottsdale Infiniti parts and get the NICO discount.
CAI for the Q has been tried and the results were disappointing. The Q doesn't respond well to "rice" modifications. Some guys have the K&N drop-in and are happy with it, but it's easy to dirty up the MAF sensor, and AGAIN, you're not gaining HP. Plus, if you can't afford $7 for a new air filter element every 6K miles, you should sell the Q anyway.
For less $$$, try the Dynomax Ultraflow dual in/dual out muffler - You can get it installed for around $200, and it's got a nice growl, but don't expect major performance gains.
Maintenance will pay off more performance-wise. Have you cleaned the TB and plenum? Fuel rail flush? All synthetic fluids? Injector cleaning? New O2 sensors? New NGK plugs? New thermostat? Checked all injectors for full function?
An early 90's Q with the JWT ECU and good maintenance will run low 15's to high 14's all day long.
Also, find out if the guides have been replaced. If not, you're driving around in a grenade with the pin pulled.
There was a member who paid $1300 for a custom, fully mandrel bent stainless 2.5" dual exhaust system with Borlas and free-flow cats. Gained MINIMAL HP, and has since removed the system and sold it.
You need to realize the Q was designed to operate on the ragged edge of performance vs driveability. There's just not a wealth of "hidden HP" that the little Japanese engineers "neglected" to tap into. You get their very best every time you mash the throttle.
Like I said before, spend a day or 3 reading the Online Mechanic - You'll know all you need to know by then.