Ask the SA is that what happened to all the Japanese Latio, that they are suffering from many problems like noise? What you are asking is to change to a stiffer Japan Spec Rear Springs, which they are using on all the Japan domestic Latio. You should tell the SA straight not to give you all the crab and waste your time.
You can challenge him to put three men behind the rear passenger seats, and see whether your Latio rear suspension will sink. Tell him you have problem clearing many car parks hump with passengers.
The Japan Spec Rear Springs is used on all Japan domestic Latio, it should be well tested and proven. It anyone worry about ride comfort or something else -
1. don't change and don't complaint
2. spend some money to install after market springs stifferners
3. spend more money to change to after makret springs
4. change all the suspensions and springs to your heart content
Tell the SA you have known that TCM had changed the rear springs FOC for few Latio already, ask him why he is treating you differently if your ride is still fairly new?