Alex,

Check out Tone's site for details on the Finnell Powervault muffler and catback. Horsepower and torque were improved on dyno testing and in actual driving. It was my VX that was tested before and after they custom built a catback system on. The documentation is a PDF file on the site.

http://tonemonday.com/accessories.htm

Very noticeable louder sound which mellows and grows lower in pitch with time. Its a nice low growl now.

Cant comment on Flowmaster but I am sure others can.