The 1st gen ECU is too stoopid to make any advantage of aftermarket add-on chips. If Nissan is going to spend the money designing a fuel control system, they are going to set it for the best balance between horse power, economy, and torque depending on what they think the target market it. About the only option is sending an ECU off to be reprogrammed based on any mods you have done. AC looks to be the only ones offering the service and I haven't read any reports of people doing it.
Also be wary of anyone saying 'put this randomly designed piece of steel in your intake and you'll get tons of HP'. While those designs worked in carb'd engines, they are useless in a direct injected engine.
Intake spacers may or may not offer any benefits. Most people say the theory is they thermally separate the upper intake from the lower. In truth, the gap is so narrow, and there is so much radiated heat in the steel box of the engine compartment that there is only a marginal difference in temperature. The other theory is they add a swirling effect to the incoming air, which, again, is not as effective in a direct injected engine. People claim HP improvements, but I have yet to see any real dyno tests.
The best thing you can do is get an Ultragauge or similar to monitor your instant MPG while at speed, and be very gentile on the skinny pedal.
