Yes it's an old thread but in the interest of continuing the topic...

The plastic intake manifold gives you more power than aluminum because it doesn't absorb as much heat which means the air in the manifold stays denser. What you need to do is to find a new plastic exhaust to replace the steel one as it too will allow less heat to get absorbed by the manifold which will mean that more heat energy is retained in the exhaust pipes to help scavenging and in turn will increase power.