mazda millenia s major power loss?

[Dennis R]

200 mazda millenia supercharged with 120k and automatic transmission. I pjust picked up the car and from the start was told that the firewall exhaust will need to be replaced. Needless to say the exhaust is loud to a damaged/cracked manifold. My area of concern is that the car has a major power loss. I am having really hard time accelerating but if I "manually' shift gears it does better and gets up to speed but the car still should be faster then it is. It feels as if the cat coverter is plugged up. What an exhaust manifold cause this? I am not sure if the firewall side has a built in cat in a manifold. I need help