Good day.

Which is better?
Should all the dowpipe be changed including the catalyst with a with a 200 cell catalyst and perhaps a bigger downpipe?
Or only the catalyst with a 200 cell catalyst?

Downpipes usually last as long as the car does. Cats get changed independently from the rest of the zorst system. 

Maybe try cleaning your cat before replacing it.

It is clean.

The technician examined it.

I am considering to change it firstly for performance gain secondlly for less heat because it is lpg converted.

Ah right. You didn't make that clear in your opening post. So you want more performance and less heat. 

As for less heat take the exhaust off and get it ceramic coated. 

As for performance and with it being lpg converted I can't comment but maybe contact Mountune as they specialise in Ford performance. 

I am not living in Britain.

The rest of the exhaust is already changed. Larger diameter and two inox free flow boxes (resonator and muffler).

Some say that with a sport catalyst it will gain 2-4 hp, others say that it will gain few - almost nothing,some others that the difference will be noticeable.