Having had both the S14a was generally about 3-4 mpg better than the Zed when running at stage 1, though could easily get 38 mpg on a motorway run, whereas the Zed will get 33 max.
Sounds like the S14a is drinking abnormally! Have you checked the lambda sensor? Unplug it see how that changes things.
Yea I'm over fuelling I'm almost certain. . Getting it mapped in September so I will run rich till then.