Can't comment on the fuel economy yet as I have only driven about 100 miles.
Performance wise (this won't be popular) it is in a different league. Possibly not as keen low down, but from about 4k it flies, getting more and more urgent as it approaches the top end. I think another reason for it is that the M feels like it wants to be rung out, whilst the Z felt a bit flat when it got to 5k. It certainly feels more exciting flat out.
Again though, I am not trying to detract from the Z, for the amount that it cost me over 12 months it was excellent.