its a twin pipe system through the cat......the only route to take and not bodge it is to fit a 525 system, with twin pipes all the way!  its not just for the look, the car needs a new system and this one was too good to miss - still had the stickers on it, and rather than buy some aftermarket jobby i've gone this route ....

i could have just fitted a 'max power/halfords special big bore soopadoopa' tailpipe trim, or done as you suggested!