Unfortunately I disagree with Mark on this occasion
his advice is well-meant but I highly doubt there'd be much gain from changing the manifold arrangements and it would be at the loss of low-revs torque. The standard exhaust manifold on the 2.4 is a multi-branch design with five individual stainless steel tubes. The standard inlet manifold is a variable-length design to give the best of both worlds. Both are pretty much as good as you can get.
However, the exhaust manifold branches do all join together at the first catalytic converter, which looks suspicious. On a four-cylinder it was always desirable to have a long dual downpipe - one pipe from cylinders 1 and 4, one from 2 and 3 - joined under the car's floor. But with a five cylinder - who knows?
maybe you need three downpipes?
Ohhhhh... reading Mark's post again, I see the word I thought was "dedicated" is instead "decated", meaning a modified exhaust manifold with the catalytic converter removed. That makes more sense now! sorry, maybe we don't disagree after all...
To make a more powerful 2.4, I reckon the most cost-effective approach would be to graft on the cylinder head and turbo (and pistons?) from a Coupe 20V turbo engine (2.0 five cylinder) - then perhaps run a slightly bigger turbo.
Failing that, there are Colombo and Bariani camshafts available for the 2.4 to do what Mark suggests.
I never understand why people think the 2.4 is short on power. A 1.2 FIAT 8v engine has 69bhp (an average kind of output) and the 1.2 16v, in Sporting form, has a comparatively excellent 80bhp. The 2.4, while being twice the capacity, has 10bhp more than twice the power of the Sporting 1.2. Where's the problem?
-Alex