Z4 Jay? Nooooooo!
I'd be surprised if you like the Z4 after owning the Zed. Had a Z4 3.0i Si Sport (267bhp model) for a few days last year (fortunately it wasnt mine) and just wasnt really that impressed - i think it was the whole Z4 experience that let me down - i cant really describe it - it just didnt feel right, it was quite uninvolving and generally felt like it lacked something though i'm not sure what exactly.
The best thing about it was the engine but it definitely had less character than the V6 in the Zed, the dimensions of the car felt wrong, the steering is a lot lighter and the handling is by not in the same league as the Zeds.
Plus the bonnet is about 50ft long
One way to improve your MPG would be to swap your auto Zed for a manual Zed - means you'd still be driving the car you want at the end of the day