I've only slightly modded the intake on mine in that there are two intake pipes for the 3.2 vectra/signum. There is a short one for the GSI and a longer one for the elite, which I have. The longer one stretches from the airbox on the offside all the way across the top of the radiator. It contains resonator pipes to keep the noise down as the elite is meant to be the refined version.
The one for the GSI just goes straight up to the grille and gives a little more V6 roar on acceleration. So I have modded my long intake by removing the resonators leaving an intake hole about the same place as the end of the GSI pipe would be.
I doubt very much it gives extra horses but it does give a meatier sound.
I've also changed the paper filter to an oiled cotton as I do beleive that these flow better when dirty than a dirty paper one.
Stainless free-flow exhaust is on my list but only when my current one dies.