Gear box change fliud on time? every 20~30k?

Usually changing new exhaust system u will having different powerband.
Most car after change whole system, specially header, the car refuse to go when hard press, or refuse to go when light press... Thats why sometimes ppl not recommended to change header / decat if u did not plan to retune your car with piggyback.

Yes, something like emanage...

Actually I have some recommended set up for 1.5 auto, for model with cat con... there are few choices.*without change header

1st : change everything after cat con... to 1 long bullet + system block type muffler + 1.6 mandrel bent or 1.75 crush bent.

2nd : Decat ( if the oem cat is stuck ), replace cat with a twist type bullet, or replace with a straight through short type bullet, other same with the 1st choise.

Technically the back pressure change your car powerband, which might made higher or lower low end torque, which will slightly effected gear change.
Exhaust gases back pressure din`t effect your gear change.. your exhaust gases will never go to your gear box marh...

For honda city sx8 larh...
I recommended to replace header with a design which same with your stock, sx8 originally comes with long type 4-2-1 if I`m not wrong.
So u just get a long type 4-2-1.

So here is the setting, Header 4-2-1 > 1.6 piping > 40cm bullet > 1.6 piping > system block muffler.

System block type muffler are those tri flow/dual flow/twin loop/chamber type.
Tri flow muffler in local they called it S-flow.

The exhaust backpressure effect your engine power... and technically the power effected your gear change.

I usually never ask my customer to change header and cat con, unless it is crack or rust, talk about cat con, i think city sx8 dun have 1.

Did u double confirm with yout current header size, and your muffler should not bigger than 30cm..., 2.0 is okie for u,
and how long was your header lenght?