Mandrel bends are better because the pipe retains its full circular shape. Pressure bent pipes will bend the pipe in a way that stretches the pipe on the outside and squeezes the inside together giving a small skrunched pipe that is worst then stock. The mechanic is right that the cat is restrictive but the whole point of an aftermarket exhaust system is to make the system as a whole less restrictive, lazy mechanics and there stupid logic, you might not want that guy working on you car.
The mm to inches conversion is roughly 25.4 mm=1 inch.
Ok but nobody here has mandril bending tools.
I guess I have to get an aftermarket catback instead of custom.
The last part was funny hehe
but I get you