You can get it louder without cats but the best thing to do is to go to mustangexhaust.com and listen to more clips. This one has my brothers slp exhaust with my flows
http://www.mustangtuningforums.com/showthread.php?t=993 , You will notice that his sounds different but its misleading as you have to drive the car to see how it will sound on load and deceleration. I will try to get some more vids of both cars. Just having the car at a standstill and reving the engine is not good enough to make a good choice. Like you said you want to hear how it sounds on deceleration which is something that I forgot to do.