better stick to 16 , just find nice pattern rims

16 to  17 , wider tire , more traction on braking and cornering , stability on highspeed ,

down side , pay more $ , increase in FC , less comfort

maybe u can try 16/55/205 or 215 with nice rims

definately can notice the comfort but just slightly increase of FC due to slightly heavy tires and wider tire ( need more power to more the car)

even my last mazda 3 1.5 CBU comes with 15" and it just enough for 1.6

it up to you,

why not u try any members of m3  who already upgrade to 17" then u gonna get better feel

If your aim is just to reduce fender gap then the right thing to do is get a sport lowering springs. If you change rims & tires from 195/55/16 to 215/45/17 then the fender gap will be only reduced by a measly 2mm, or practically not noticeable reduction! Because the difference in radius between the 2 sizes are just about 2mm. On top of that, since your tire is much thinner, it will actually look more ugly and odd since the fender gap will look larger compared to the thin tire. then of course you'd get much reduced comfort, and also FC (this depends on how heavy the 17 setup will be).

So my advice don't change to 17' but get a lowering springs, or if got more budget get adjustable coil over.

get 17" and 205/50/17 tire
original m3 using this size as well , fender gap reduce , rim bigger 1inch , sidewall still can remain thick

after rim change only decide lowered ornot

Well, you're wrong.
195/55/16 diameter = (19.5 x 0.55 x 2) + (16 x 2.54) = 62.09cm
215/45/17 diameter = (21.5 x 0.45 x 2) + (17 x 2.54) = 62.53cm
Difference in diameter = 0.44cm
Difference in height/radius = 0.44/2 = 0.22cm = 2.2mm

Or you can just compare here
http://www.1010tires.com/Tools/Tire-Size-C...55R16/215-45R17
Lowering springs will depend on the brand. If local one like GAB, can get for maybe rm400 or even less.
If going for reputable brands like Tanabe, Tein, Eibach, then expect to pay rm1000-1200.
Not sure about Taiwan brands though.

for TS case , after rim change if the fender gap are acceptable then y still need lowered?

most people practice it. Back to factory ones  before claim any warranty.

When changing rims & tires actually should always try to maintain the same overall diameter with variations less than 3% compared to std setup. Changing to 215/50/17 will increase diameter by >4% so this is not recommended since it will make your car noticeably slower and braking also longer. So changing rims should not have any significant effect on fender gaps. Therefore if want to reduce fender gaps the only proper way is by lowering the car, and that has to be done first before choosing rims & tires to make sure that the rims & tires you choose would be perfectly fit with the lowered ride height.

I don't know about local brands quality, never used before. But GAB seems quite popular.
Yes, it can void your warranty with regard to suspension parts.

Yes, many bolts already have markings from the factory so it would be obvious if already opened. Then just don't do anything, not even change rims as it won't reduce your fender gaps and it would make it look weird IMHO (due to still big fender gaps but will then coupled with thin tires). FYI, even if you only change rims, the SC can also make it as an excuse to void your warranty since the rims are not as per std spec. They can say that your rims are heavy, not the correct offset, too big/wide, etc hence makes the suspension works harder (which is true to a certain extent).

Air filter they won't be able to detect. I've also changed with after market ones, and every time I come for service I put the original back. But in time I think they would suspect why the ori filter never gets dirty!

As for the choice of rims, I'd pick which ever is the lightest one, so bring a weight scale when you choose your rims! That's what I normally do.

I'm currently driving Toyota Mark X 250G.

Don't get your hopes up on air filter, there'll be no noticeable difference! I only change just to save money cuz don't need to change anymore, just keep cleaning it. If got any small gain it's just a bonus for me (but unfortunately no).

Currently many TW rims are already very good quality and pretty light too. Std alloy rims are heavy so finding lighter ones than the std is very easy these days! I suggest check out Advanti as it's using using technology from Enkei so it's good. Another decent one is Lenso, also got good designs and relatively light.

Rays is one of the best & expensive rims but I'm talking about the original Rays, not those replicas. I don't know how good those replicas are,  maybe sufficient for normal daily use.

Advanti is cheap, also made in Taiwan.

No other way if you're talking the proper way! Either change to sport lowering springs (which is the cheapest way), or adjustable coil over or air suspension. The other cheaper 'ways' or 'tricks' like cutting/hot pressing/modifying original springs or using springs meant for other cars are the wrong way!

neh nestum deals with a lot YHI products, basically Yokohama/Advanti lol

that price for original, i highly doubt.