Surely 15% was plucked from the air?no, for my masters i worked on the instrumentation for a mechanical engineers project which involved strain guages. One of the papers I read as background was someone had put strainguages in various bike parts to measure efficiency.
The biggest losses were in the soles of trainers (they used normal trainers not skate shoes which have less padding underfoot), and the 'pull' in SPD's was negligable under normal conditions, even riders who thought they were riding prefect circles and producing very even power outputs were infact still pressing down slightly on the upstroke, the up only comes into play in a sprint or mashing up a climb and even then was only about 5% of the power IIRC as the power is limited by your cardio system, so using different (i.e. you can use the oxygen/energy to pull up, or push down, not both) muscles was no better.
So flat pedals probably lose you about 10% in a sprint and 5% elswhere. More if you use unsuitable shoes.
The Peaks having no corners so out of practice
Is true, I moved away from the Peaks after 4 years there and couldn't corner for toffee! I could ride down steep rocky chutes on a rigid bike all day long, but corners were ailien!