From what I can tell, Zex will only allow the fuel pressure to rise when you have the system on otherwise it will let the regulator run normally.
The SAFC should not interfere with the zex as it is a maf scaling device and that indirectly affects the injector duty cycle instead of fuel pressure. So bottom line, an SAFC that is used to fine tune your AFRatio on the car will be operating the same way even with the zex system on or off.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
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'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black