We cut a stock pipe in half for some closed course races where there is no sound requirement. (private land) It gave the 300 quite a noticable shot of snot across the powerband.
I would NOT recommend it for public land use. Not much louder at idle, but when you hit half throttle it cracks and can knock the needles off the jackpine. (Aside from the fact it has no spark arrestor, doesn't meet the USFS db requirements and is highly irresponsible.)
It does improve performance, but at a very high price if you're within earshot of anyone.
Maybe we'll slip off the Turbine Core II, slip on the modified stocker and have it sound tested at the next race just to see how loud it really is.
My bet is at half throttle it's over 105 db.