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
'07 450EXC, '06 950 Supermoto, '09 YZ450F