Two things have happened since the 156/GT days - the engine has been strangled by emissions regulations, and the car has got heavier. My father in law has a 1.9 GT, no DPF, no EGR and if you drive it like a saint (65 on the motorway) it can do 60 mpg. If i thrash it in my usual style, I can get it to about 40, but rarely below. "Normal" driving gets about 50.
Our 159 SW is a 2.4 - remapped, and with saintly driving, I can get 47 mpg indicated, which is probably about 43 in the real world. Thrashing it is far more consumptive: it is a much heavier car than the GT, so over back roads it suffers badly - I often see sub 30. As soon as you hit the motorway, economy is far better because you are not accelerating all the time.
i would strongly recommend the re-map - think of it as a upgrade to driveability that will also improve the MPG a bit. Our 159 feels like it is a attached to the horizon with a bungee when you floor it.