Mine was EXACTLY the same. It had a new CAT in september and was the exhaust blocked up. I paid £84 to ATS for a new one and it feels like a different car! I had bugger all power and it just would not rev above 4k. Now it's smooth through the rev range and 100% more responsive.

Mine rattled on start up - the bloke that did it said the middle box was completely "mullered".

Exhaust - bet ya