As per above, you're not going to see a big jump in HP - the factory has already picked all the low-hanging fruit. Off the top of my head, 20-30 extra HP seems to be what most people are reporting with similar mods to yours (and a tune). A lot will depend on who does the tuning and, as per JLarson, the dyno (and operator) will have an effect.
For many of the DDers on this site that have their Zs tuned, it's seems to be more about responsiveness than actual HP gains. The engine just feels peppier. Apparently there is some throttle lag built into the factory tuning.
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