My bikes stock tyres.....few times when I was cruising up flyovers on a rainy day,  no sudden acceleration or anything.. Just maintain throttle , the rear tyres spun and slipped when it goes over the white painted arrow on the road.  Could be because of my  bikes high torque.

Can be a bit scary.

If smaller and shorter mirrors doesn't serve the purpose....forget about it.

Safety first.

It's not too bad but it still is noticeable in the situation I mentioned (those white lines). The bike can still maintain decent speed (90~100km/h) in wet conditions. Just don't expect it to ride twisty roads all too well.
My solution for aftermarket mirrors were a bit costly, but I went for the option that gave the best safety possible. I didn't want to get the widely available tiny copy Rizomas and such since they were too small, so I ended up getting a Bike Master Ninja 250/300 2013~2017 OEM set from EBay. The mirrors' base are the same as new Ninja 250 and 400, so they screw on with the same screws, but they are about 5~6cm shorter than the standard ones. With some minute adjustments I was able to get good view out of them. I don't recommend doing this if you have wide shoulders though.

Fender eliminator can pass JPJ roadblock - probably with some sweet-talking depending on how strict the officer is - as long as they are the type that reuse the license plate light and signal from the original tail section. License plate must be illuminated, the angle must not be too extreme to the point that cars behind you can't see it, and the number plate must be the standard size issued by JPJ.

I got my bike at XL Motor in Seremban.
Can see my solution for the mirrors above. You sacrifice a bit of rear view depending on your body width (10% or so in my case, not a big issue), but the mirrors are fully functional. Many people who own the bike in the USA have also done the same.

The problem with the Ninja 400's mirrors for me was their height being exactly at the height of most cars' side mirrors. It was very difficult to adjust during filtering to not hit those side mirrors, especially the family carrier cars like the Alza.
If you can live with the side-mirrors then great. I just couldn't, haha. 

My previous Kawa bike was a Z250SL, which did not have the neutral finder. So it was a pleasant surprise when I found out how easy it was to shift into neutral on the 400.

EDIT: Added image for side mirror length comparison. Longer one is standard, shorter one on the bike is the replacement.

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