Ok here is my honest feedback. based on my years of experience in this field.

1. Muffler only adds a rough amount 0.5-5 hp to a bike based on the CC of the bike. (ciplak muffler only makes sounds and also maybe decrease in hp)
2. Full system adds roughly 1-15hp again depending on the cc of the bike. (forget about ciplak)

now... here's the cold hard fact. apologies if this sounds harsh a bit, but note that no pun is intended.

250cc can only go up to a certain level of HP, you can feel the small and subtle differences when you upgrade this and that, but the amount of money you need to fork out, is simply kerja bodo. further more when you sell the bike later, the price effect is not much of a difference. so best to leave it as is.

I would rather recommend that you look for a higher HP bikes, you get the best "bang" for your dollar.

but if there is a possibility that a modified 250cc bike can beat the shit out of a 1000cc bike without costing almost or near the price of the 1000cc bike, then your investment is surely worth it. but then again ada ke?

Not to discourage your intent for NFS (need for speed) feeling, i've been riding for way too long and seen too many unnecessary problems caused by aftermarket parts that only helps a bit. running the bike as is, is more cost effective as all items are design for efficiency. again 4k for a full system? might as well sell the 250cc+4k is almost half the downpayment for a 1000cc bike or even pehaps a cash buy for an older model 600cc-1000cc.

my humble thoughts.

no la bro... just the truth, you know me

There are some shops you can trust, but majority of them nowadays are just to make a quick buck out of you, sadly.

Correct, those are what you should spend money on, gears, protectors especially for you and then your bike, in that order. The next best thing you can spend money on will be your tyres (good tyres helps a lot during rain, coners and especially braking).

Melaka to KL is not that far bro, with the standard Z250 you can reach in about 1 hour. it's good enough because any faster, you'll get a summon... which is so not worth it.

I kena before dy with my Cagiva doing 180kmh++... but I didnt pay immediately and went to court. To make story short, I only pay RM 30.

yes, it does but too little to feel... the most you can feel will be at gear changes... smoother, easier and less noise.

yup, i do it in 1 hour

better you go for the K&N air filter & good spark plugs... more horsies gain rather than the slip on and lesser cost... once you done these small and essential upgrades... if you still want a little more, then go slip on...

not sure about the 250's but for bigger cc EFI bikes, if you change the air filter and exhaust, you might need to remap your ECU. otherwise the performance will sux.

NGK has 3 types of plugs, the normal, the type R and the red color box for race purpose. try check if they have the red box type for Z250.

On my own personal test from majority of brands, I find that Silkolene & AGIP are the best.

Honestly speaking, Motul, Shell and a lot of good name brands has knock off... do buy from a trusted workshop. I assume you might kena the knock off or your engine got leak or burning engine oil.

Now, some people might disagree when I say this, the best SAE rating to use in Malaysia will be 15w-50. This SAE rating would be just right for our weather. 10w-40 is too thin and easily burnt off... while 20w-50, you might need to start you engine a couple of time during cold. If you are using a V-Twin engine, there are new oils which have XXw-60 rating, this will be the best choice but if you are using Harley then get the 20w-50 specifically designed for Harley V-Twin.

Please note I have a lot of bikes and no car. hehehehehe

but to me best to use original because afraid that the ciplak one might decrease engine performance, unless you tested the ciplak one before and got umph.