Not sure, is it the main cat or the one next to the turbo ?
My first through would be boost leak, as it's spiking, but then dropping right down, like a leak only when on boost. Do you have access to vagcom ? or a scanner tool ? might be worth looking at the fuel trim if u suspect under/over fuelling. Also check for 'soft' errors, i had a knock sensor feeding bad data which was making my engine pull my timings right back...