I currently have the HKS Hi-power with a helix DP and test pipe and am thinking about going with either a megan catback or ams to get the full 3" and was just wondering how much louder each of them would be than the HKS....

 

The HKS has slight tapers, so AMS/Megan will be a bit louder, but I think the AMS will be louder than the megan.

Sound clips/DB readings vary greatly on where they are marked, windows up down, rpms, etc., so just get a ride with somebody local.