Mods for NA Sport
 

No ECU tune needed (don't think one exists for the GE) unless going forced induction.

Honestly bolt on parts don't do much. Intake will make more noise and increase throttle response. Exhaust/headers will increase noise but not add much in the HP department. I'm sure those things do add some but its not much. Best way to make power is adding a supercharger kit or turbo.

Summer tires and coil overs and you will have a fun go cart!!

If you are doing bolt-ons, the only way you are really going to see more power is with a Hondata and tune. Of course, FI will increase power but you'll probably have to piece together your own kit at this point.

I agree with GolNat on the tires and suspension upgrade. Maybe a rear swaybar too. Before you dump money into that though I would make sure all the ball joints, strut mounts, endlinks and engine mounts are in good shape, replace as necessary.

You're looking for a noticeable jump in power NA, you'll be doing a good bit of digging around with Hondata (which is VERY user friendly) to get the most out of those bolts ons and it might get a little on the expensive side to be honest. Especially if you're going for the 105 WHP and up mark naturally aspirated.

Bang for buck power wise... one step colder spark plug (like Denso IK22), a decent header and catback should put you really close to 100 WHP on a GOOD fuel and ignition tune on 93 with the stock intake. You can attempt to find a better flowing intake than the stock airbox but the factory airbox actually flows decently well. Even Spoon Sports used a lightly modified version of the stock box in their racecar to be honest. Other intakes like the AFE Stage 2 for example might make a pinch less peak WHP but will pick up noticeably more WTQ lower in the RPM band. I personally have gone from a baseline of about 92 WHP (untuned with the following bolt ons) to 104 WHP on my automatic sport by simply doing fuel tuning on 93 octane via Hondata, one step colder plugs, T1R header, Megan test pipe, and Spoon Sports catback... Still had some tuning left to do on the ignition side of things but the car was lively enough for my liking at that time before I went with boost. If you'd like to see details regarding that, feel free to check out my thread linked in my signature below.

Most efficient bang for buck in terms of overall FUN wise though... Address any bushing and/or ball joint related wear items first like mentioned then find some lighter wheels (15x7 Advanti Storm S1s for example... 'Po' man's RPF1' I call them... $130 a wheel shipped @10.6 lbs. or Tire Rack Motorsport C1 if they're still available... $100 a wheel, 12.6 lbs each), a little bit wider and sticker tire (205/50R15s), put a Progress rear anti-roll bar on and decent springs and shocks (maybe even coilovers if you're up for it) then enjoy the car as the little fun momentum car it is.

I own an '08 Sport AT and while I wouldn't consider myself too knowledgeable, I personally felt my bolt-ons made a difference for me. I have an axle back, CAI and Hondata (got it for a steal!) with a base map tune for the time being. It's definitely made a noticeable difference on the low end torque for me.