Basically to get the most out of your N/a you need to make sure it breathes better.
So that means:
Intake mods... don't be fooled into thinking a ram pod is the answer, better flow into a standard airbox will do for an N/A. I had a secondary air feed into my box with a Blitz filter and it worked well.
Exhaust mods. On my N/A I had a set of fujitsubo stainless steel extractors, which were mandrel bent with equal length runners... flowed like a mofo.
Also got tuffy's mufflers in epping to make up a 2.5" exhaust with a good resonator and muffler for me, so it wasn't loud, but flowed well.
Get a good set of spark plugs, and have the car tuned on a dyno.
Other things like regular service, oil changes, oil filter changes etc make the car run better.
At the end of the day if you're restricted to an N/A... power to weight is king. Get yourself a nice VTEC CRX or civic or a corolla (maybe the classic AE86 if you absolutely need a RWD car or an imported AE111 Levin if you don't mind FWD) with VVT 1.6 N/A and you'll have a faster car than a heavier skyline, even if the engine is smaller. And with the fuel prices these days, a smaller engine is actually a good thing.