Camshafts are far from my cup of tea, but looking at the numbers, you can see that the Clio's engine has less peak torque, and at a higher rpm (215Nm @ 5500rpm for the clio and 245Nm @ 4500rpm for the boxster).
To achieve the same horsepower from a smaller engine, the clio has to have more aggressive camshafts profile that relies more on the scavenging effect to make power.
While the boxster relies on its displacement and not so much on the scavenging effect to achieve the same hp numbers, but with more torque and a wider powerband.
The clio RS also has a very good header from factory too compared to the boxster 2.5!
Also, I think that most headers available for the boxster are too big in diameter, that's why they don't show much gain. The size of the primaries on mine are as big as what I usually see on big V8s.
If I remember, on the 101 projects book, there is a 2.5 with camshafts that makes around 240hp? I have to check!
Feel free to correct me if I said anything wrong
