The cat is after the flexy pipe, looks like a small silencer. as your having the exhaust replaced do the cat as well. The v6 mondeos have three cats in total that main one and one in each manifold, which means if you bin the main cat the other two are enough to keep emisions low enough to pass the mot . Loads of people on MEG have done it. As for which system, apparently blueflame release a bit of power and are quite often on flea-bay, but personally i don't like the tailpipes. I have a custom made powerflow, and prices range depending on the place that fits it. Mine was £282 all in at a place in redruth (cornwall).