The flexipipe is not a CAT replacement, it just removes a 42mm restriction ford have put in the exhaust system (only reason I can think for doing this is to keep power below 150bhp to reduce insurance group). Replacing this is said to increase power in the region of 7-8bhp. I don't think I'd want to remove the CAT, too much hassle comes with this for me!

If it was changed then yes you are right there would probably be an issue with the sensors, but somebody on the 'other site' has made a device to sort the management light problem when the CAT has been removed.

Chris