On a 79 300sd I replaced the final muffler with straight pipe with no discernable increase in noise. I drove it for a short while with nothing but a couple feet of pipe behind the pre muffler and you could tell the volume then probably because the exhaust vibrations were under the car. This noise increase was very slight though.
If you went so far as to replace everything with a straight back pipe I doubt the noise would be any louder than the engine. I am not an expert but I also would doubt that this would lower your backpressure to the point of causing problems. If the work has to be done anyway why not take the opportunity to have fun "hot roddin" your 240D with a "performance exhaust" and put at most a gutless muffler at the end.