I tried one on my F650GS. Very nice sound without the db killer, just a tad louder than stock. NO backfiring at all (stock air filter).
With the db killer installed, it is quieter than stock.
I've since sold my bike and my Leo One is for sale in the flea market.
I paid $330 + shipping from MotorcycleSuperstore.com
2007 KTM 450EXC Plated
2006 KTM 950 Supermoto
2009 YZ450F "Woodserized"