Works out around 25mpg by my calculations
and comparing that to the readouts i'm getting on my ScanGauge these days that is spot on.
Oh and, as for changing... IMO it's best to do what suits you. After all it's you that has got o pay for it, run it, and live off what you earn.
If i was to be totally honest... the Astra Nürburgring version is tempting.
(but that is 20 trips to the US
)