Where did you read that??
The engines are identical, so in theory there should be no difference. In practice, engines manufactured by those such as Renault and Peugeot can vary quite a lot in terms of output (not by as much as 10bhp though).
There are many stories of under/overpowered Clio 16v/Williams/172s. Moreover, the whole process of measuring power is full of debate and inaccuracy. If you take an identical car to two rolling roads, you may end up with vastly different power figures - and thats before we even enter the vertitable quag-mire that is calculating at the flywheel power figures!
The starting point for tuning is the seminal combination of a free-flowing exhaust, chip and maybe an induction kit (which can have a negative effect in an engine bay as small as the Clios). Wont net you much extra power, but will work better the deeper you get into mods territory - Im talking about head work, cams, throttle bodies etc...
