Could we have another b11bob on our hands :p
In all serious ness if it hits 140 mph then its not down on power.
By all means try the fuel filter, but I suspect the rolling road wasn't very accurate. Have you got a friend with a standard '6' whom you could have a comparison with to see if the car really is slower than it should be?
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"Supercharged - 454.1bhp/317.5lb/ft"Peugeot 306 GTi-6
2000 (X), Moonstone