not much!
Just fitted a 45-54mm titanium half system made by Sil Motor, and was surprised (almost disappointed!) at the quietness of them compared to the Termi half system I had on before!
Quality is a lot better than the titanium Termis I've seen - the weld on the sleeve is very neat and uniform, rather than the mess on the more expensive make.
All in all, for £425 shipped from the German importer, not too bad
Just need to find out how to make em louder!