I totally agree and my Z is certainly not a weekend car, I use my Z as a daily drive, that's why I got a high mileage one.
You would soon get bored of the Throttle Controller on Econ mode and stick it on Sport believe me I normally have mine on Sport 5, the other week it was raining hard and I did the sensible thing and put it on Econ... I thought I had just left my engine on the Dual Carriageway
All I was saying was that according to your calculations you can have 3k a year in your pocket, thats a fair amount and worth considering. Instead of a repmobile how about a sporty diesel, either a hatchback or even something like a 2.2l Jag diesel.
At the end of the day the choice is yours, only you can decide, there are also other considerations not just Fuel, a Z clutch can only be good for 30K so that would be almost one a year, service would be almost 3 a year etc etc it all adds up.
Good luck with whichever way you go