How can I fix the poor running of my Rover 45?

My car is a Rover 45 (16IL, Man.: 2001) and has got poor running (If you push the accelerator pedal in higher gear suddenly until the floor then the car is getting deep voice is like that somewhere loosing the pressure, no accelerate and the car getting slow). I could hear scratching voice from the exhaust system.

Firstly, I took to a garage to find out what might be the problem...the garage said that the rootcause is my catalyst was broken. After that the car was tested on MOT and all the result of the test was OK and normal.

Anybody can give me any advice?

Thank you for your help.

Adam