Our Mercedes Sprinter van at work had the cat/dpf stolen, they just unbolted the whole exhaust and did away with it. Must of taken them about a minute, was only held on with 3 bolts and a few rubber mounts that they chopped off. Merc wanted £3500 for a new one, and insurance wanted to write the van off
We fitted a cheap shitty aftermarket cat/dpf, gutted it so it was hollow and had them mapped out. Never happened again, thieves don't want aftermarket cats only oem ones as the cheapo ones have bugger all precious metal in them (hence them being so cheap)
As others have said though, celica ones aren't worth much and not easy to access so they won't bother. But decat it anyway