Hi Kai,
I'm an owner of a Neo (H-Line) Manual and I would like to share my personal experience of making the engine 'lighter' and more efficient. I've been working on my car with a friend who is a car-nut himself. Basically, he has done a lot of trial and error with his car and so far, the improvements that we've applied on my Neo has been forwards.
Anyways, the changes I've made are:
1) Open-pod air filter with a custom aluminum intake ram.
2) Discard the 'cat' converter in place of a 'silencer' [not sure wut it's actually called but I didn't put in straight pipe to ensure some back pressure for low-end torque]
3) Ultra-racing anti-roll bar
4) Zero one Lightened flywheel
5) Hankook Ventus V8-RS 205/45 R16
Ok.. the impacts however, are not being dyno-ed but rather has been based on the driving feel of the car. I know there are ppl who might think of placebo effects and so forth but let me tell you that there's a fine line between placebo and significant changes.
Mod 1) - improved the engine 'breathing' hence better acceleration
Mod 2) - same as mod 1) but for the outlet and it makes the engine smoother to some extent
Mod 3) - stiffens the rear end and allows me to tackle certain sweeping corners faster and more confident (e.g. some corners which I previously dare to take at 130km/h can now be taken at 150km/h or 160km/h depending how much adrenaline is in ur body

)
Mod 4) - THIS IS REALLY REALLY LIGHTENING. This mod is worth every cent but the effect will wear off once you are used to it. I kid you not, upon changing you feel like your car jus recovered from asthma.
Mod 5) - I love this tyre although if I ask different ppl they will give you different brands. BUT wutever it is, I believe it's leaps and bounds better than you stock Contis. The difference? Well, the contis for one are twitchy in the wet. This has a more assured feel to it. Hence why I love it so much. In the rain, the car feels like its on track. Another thing though, it's a soft compound.. hence the grip but I noticed that I changed the contis more often than I've changed this.
So, there you go. These are the mods that I've done and I believe that it has been very rewarding to drive the Neo. Of course, I wouldn't kno how it'd feel to have a transplant but then again, if you have a car with a fake soul (i.e. Mitsu engine), it wouldn't be a thoroughbred would it? Haha.. but that's just my take.
I'm also planning to change my brake line to a steel braided one; the discs (god knos that the discs is really awful for the Neo) to an aftermarket discs; extractor and muffler which is expected to release a few more horses. I think with all these in place, all you need to do is invest on ur driving skills. Another advice, do your mods in stages so that you'll reach ur limit b4 u upgrade the car. I've seen a lot of car being mod for the sake of it without being driven for its mods purposes and I'm not talking about cosmetic mods.
Have fun with your neo. Car's a bargain and it's fun to drive!