I have Flow 40s and a MAC o/r H, love the sound. Loud when you get on it, quite idle. FYI I had a SLP LM1 Catback installed and couldn't stand it. I didn't want to go back to my stock mid so I had the flows welded in
If you are considering an exhaust change from your MAC mufflers go with the 40s, you wont regret it.