If the scene has less dynamic range than either camera - as it is common in the UK - then absolutely nothing.I shoot a landscape at ISO 64. I'm losing how much DR?
No mechanical shutter using a tripod can put the Z8 a little in front.
If taking portraits the new Z8 skin softening ability might also put the Z8 a little in front.
To "justify" going from a Z7 to a Z8 implies usage other than landscapes.