Minor point - but the term "straight through" in relation to exhaust boxes typically means a box/silencer where the pipe runs "straight through" i.e. if you hold the exhaust box up and look through it you can see out of the other end. In the middle the pipe will be honey combed and the box itself will contain wadding to absorb noise.
The alternative is a baffled box where the entrance and exit pipes are in parallel and held in place by baffles and the gas doesn't have a straight through path.
Typically center boxes are straight though to provide minimal flow resistance to fast moving, hot gasses. Rear boxes are more often the later type as the gas has cooled a lot by then and is more dense and moving slower as a result - so the later design is used as it result in better sound reduction.
So removing a straight through centre box and replacing with a length of pipe will often have little affect on performance, but will as already mentioned above give you a much sportier sound.
Now - 156 Turismo
Gone - Merc C240 Sport (RIP), numerous VXs, MGB