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To my knowledge 90 should not make any more power unless the ECU can specifically detect it and adjust ignition timing for it.
Typically engines with high compression can have knock when above ~80% load, and the Insight has pretty high compression. It also has a knock sensor. If the sensor detects knock, it'll pull a few degrees of timing, which affects both power and fuel economy. I believe any knock detected also throws a CEL but I'm not certain if there's a threshold (above zero) for that. But, typically, an engine will not knock on the octane it's rated to have, and if it's not knocking, and the ECU isn't pulling timing to get rid of the knock, higher octane shouldn't help economy because it only really provides more resistance to knock.
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