I'm surprised you only got 31mpg if you kept below the vis band. My average of 320 to a tank over a commuting week was a mixture of driving and that did include some heavy use
However I will say that the remap has made quite a difference to the fuelling and the induction set-up makes a huge difference as the air flow is much improved. Don't be fooled into thinking that because it is modified doesn't necessarily mean it will use more fuel. It is actually more efficient and can be driven as such.
The OE set-up is only a compromise and is not optimal. Afterall the ZS was not initially designed to accept such an engine so the ancilliaries had to be squeezed in.
I must be fortunate and have a very healthy engine and set-up.