This is not a debate between, flow,borla,slp etc etc. This is not a debate on which one sounds better.

Simple question between all the 3 inch manderal bent systems, if dynoed on the same car, would there be any real HP difference anymore than say 1 HP? I just wondering.

 

Yes there will be, but not enough to make a huge difference

 

if your benchmark was with the same car and only included 3" catback systems then you would have a good model to base this on, But I doubt any manufacturer out there has tested the majority of systems against each other. They are all bias and advertise how they produce more hp than other systems which is not entirely accurate. I seriously doubt anyone on here knows other than there own systems which everyone has a different car/setup, therefore you lose any real benchmark between catback systems.
If you want guesses and estimates then you asked the right question due to the fact that your question relates to 3" catbacks only, not duals, no cat setups, yada yada yada. The differences will mainly be in the mufflers, blocked, not blocked, etc, so it's hard to say. I'd say you might get a 5 hp between the majority of catbacks out there....maybe.

 

The Loudmouth should be the best flowing cat back out there. Just loud pipes. I forgot where I read this but the LM flows twice as good as the next best flowing exaust which was the magnaflow. I'll try to remember where I read it from. Don't know how true it is though...