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You guys seem to know what you're talking about, so how about this - why do they sell the mid-range octane gas at all? Is it just a lure for people who think their Geo Metro will really have more oomph with a little higer octane?

Two possible scenarios, one of them being my own. I recently built and installed in my 1982 suburban a 383 CID engine. You get this by putting a crankshaft with a 3.75" stroke (same stroke as the old 400 chevy engines) into a 350 chevy block. With flat-top (not dished) pistons and stock low-compression heads, it has about 9.2:1 compression. I also put a low duration/high lift cam in this engine for better low-RPM torque. You get more torque from higher cylinder pressure. Higher cylinder pressure makes premature detonation (pinging) more likely. I could retard the ignition timing to make pinging less likely but then I would be throwing away the performance potential of the engine. This engine will not run well and perform well without pinging unless I use 91 or 92 octane premium gas. It has a carburetor and no computer-controlled anything so it cannot automatically adjust to pinging or other negative conditions.
The other scenario is with a more modern car that has a fully computerized fuel and ignition system with the potential to develop high cylinder pressures. The Geo Metro engine as it comes from the factory might not be be able to develop enough cylinder pressure under any fuel flow or ignition timing advance to require premium fuel to avoid pinging, but other engines might. As was stated by a previous post, the detonation sensors that are part of these computerized systems will adjust to avoid pinging. What they will do is to retard ignition timing or cut back fuel flow automatically, thereby reducing cylinder pressure and the performance that MIGHT be possible if a higher octane fuel was used. I think in many cases that people do buy higher octane unnecessarily. I also have a 1995 suburban with a fully computerized ignition and fuel delivery system on a 350 engine and I only burn 87 octane regular unleaded. It never pings and performs almost as good as the 383 in my 1982.

Is it just a lure for people who think their Geo Metro will really have more oomph with a little higer octane?

Pretty much.