FYI removing the cat on mine reduced power due to a loss of back pressure
First a question - You do have a turbo diesel don't you...?
If the answer to the above question was yes, then who ever told you your car lost power because of the loss of back pressure due to the removal of the cat is a complete idiot and knows nothing about how your car works....
The exhaust on a tdi has no influence what so ever on the back pressure on the engine, the back pressure is provided by the turbo, after that you want the exhaust gases to escape as quickly as possible to help keep EGT's down and allow the turbo to flow efficiently.