My Miata is a '94, not OBD2 like your '97. Perhaps that's why it takes the 10% hit. It also used to ping more readily on E10, before the Randall duct eliminated pinging entirely by reducing intake temperature.
Modern ECUs that have flexible fuel and timing maps and a knock sensor may indeed see only a 3% drop in mileage.
The ECU has no direct way, (to my knowledge) to determine that E10 or E15 is in the tank. Even with electronic throttle control, and so forth, about all it can do is sense the power needed, adjust fuel and air, and go on. This assumes that the computer does have the ability to sense power and load. Some cars do have the sensors to directly measure load, and others do it based upon an indirect method.
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