Actually its the otherway round. 50mm pipes are essential! and a Durso off of that works like a charm. I hardly hear anything from mine unless i stand right next to it.
Hmmm, interesting. I thought I had the answer. I suppose a 50mm hole can't hurt. Unless one fishy decides to go for a ride

Won't you post a pic of your overflow on your build so I can have a look. Shot
It makes so much more sense when you can visualize it 