Well, there's no doubt that in the traditional world, tuning (especially for spark advance and AFR) matters. What's at the heart of this debate for the Z, though, is whether our ECU's ability to dynamically tune spark advance (via sensitive knock sensors, mostly, I believe) and AFR (via the fast wideband sensors the ECU sees just in front of the cats combined with the MAF sensors) is so good that dyno-tuning for advance/AFR is unnecessary for bolt-ons.
Spark advance I believe was well-documented by UpRev to be basically self-adjusting even when set to insane values. You could adjust the target AFR map without a dyno if you want the ECU to attempt a richer or leaner AFR via its sensors at various pedal/RPM points on the graph. That people see gains or AFR changes when they do a quick dyno->mod->dyno cycle could just be that the ECU hasn't had time to adjust yet, and would've arrived at roughly similar numbers eventually. That people see whatever changes on waiting longer between the dyno runs and/or mods... once you wait a while conditions change and these are all relatively small diffs we're talking about.
Personally, I'm inclined to think it's pretty difficult to take this engine anywhere with bolt-ons that the ECU can't adapt to over time, although tuning the fueling maps and/or spark maps on the dyno will keep you closer to optimal right after each ECU reset so that it takes less time to adjust, and tuning the AFR targets without a dyno might be an optimization that's worth it depending on your setup.
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