+5dB is quite a lot more noise than stock.. Showing my aged and faded memory so apols if incorrect but I seem to recall a 2.5dB increase is a doubling in noise/volume o/p (cant be h'@rsed to goodle it for accuracy).
As everyone is saying if you want quiet stay with stock. Anything else that is lighter is going to be louder. The Yam option Akras (with cats) on my XT660Z were a fair bit louder even with the dB reducers left in.

Leo Vince cans are light and very little louder than stock at low revs, they do "growl" when the revs rise. I have just ordered a pair for my XT660X, I have already fitted one onto my XJR and it sounds really great so here's hoping!!! The weight saving is more than 3kg http://www.leovince.com/it/catalogo-...660r_2004/7762
Ciao Simon