If the 250/350 silencers are similar to the 175/200 items, they are fairly close internal copies of the OEM pipes, and have a conventional chambered baffle system, rather than being straight through absorbtion type / glass packs, as found in the Dunstall pipes.
Again, if like the 175/200 pipes sold by DS, they are slightly louder than stock but not noticeably so.
I'm having a senior moment, can't recall his name, but one of our members had a close relationship with the UK company that makes / made these pipes, as sold by DS and a couple of other UK bike shops. I remember him writing about how he bought the pipes unchromed so that he could tidy up the welding on the clamshell seams, make them look more like OEM, before getting them chromed.