I have been driving a 2016 Alto for the past few months, and it has been pretty good so far.
I am also mainly driving within Colombo and no issues with that.
I have driven on the Express Way, when I picked up the car from Hambantota (yes, went on the importing route). It was able to handle the 100kmph speed without any problem. We were only 2 large adults, but I'm sure it'd be fine with 4. There is a fair bit of floor noise, as with any small car. I have driven few different large Toyota's on the express way and this wasn't too bad comparing to that, may be the back seats are a bit flat.
With my own calculations, mileage was about 20 on the highway, and now doing about 15-17 within Colombo.