Dunno about everyone else, but on a recent trip to heathrow and back, i returned 54.4 mpg. It's a 177 and only 1500 miles on the clock. generally around town it returns about 45-48 mpg. This is the first oil burner i've driven that actually likes to be revved past 3000 rpm, there's some poke up there.
Haven't driven a 163 so I can't comment on the difference.
Has to be said though, if you are used to the thirst of the celica, the benefit from moving to a 325/330d should be sufficient to put a smile on your face anyway, and I can't imagine how much fun one of those is.
Save your money, buy the biggest diesel engine you can afford.
You know it makes sense.