Can't say what % but it's noticeably louder but not annoying at idle with mean, rumble tone.
And before it's louder or not, first thing you notice is the different type of tone (mean, rumbling compare to gentle note from the stock). To me, I didn't want to go much louder exhaust but wanted to have better tone so I picked up Dinan.
X5M: Dinan Exhaust | Dinan Suspension | Velos Tune | BMW Perfomance Splitter | Lux V4 | Euro Mirrors | H&R 12/25 mm Spacers | IND Cosmetic Package
PAST: 08 E90 M3 | 08 E92 335i | 03.5 E46 M3 | 99 E36 M3