My (Canadian) '06 EC300 came with such a pipe in addition to the non-catalytic pipe. As a spare, I modified the cat-pipe as you described; I scored the pipe perpendicular to my cut-line, sawed through the pipe, removed the catalytic canister (about 6cm in diameter and 7cm in height), realigned the pipe pieces using the score-lines, and then welded everything back up. I also removed and plugged the other smaller (5mm) pipe near the exhaust flange mount. Pretty obvious and easy.
The "mistake" I made was that I made my cut too high on the pipe and ended up cutting through the cat-canister; that's hard material and took forever (even with an electric bandsaw!
).
Now after a little work and a splash of paint I have a spare pipe waiting for me in the basement.
