most mufflers are packed with fiberglass, i can imagine the varex one would be to different. Personally i have no experience with the varex kits, but I've herd cars with them and when the internal valve is closed i wasn't a fan of the sound, deffinatly quiter but.
I would rather have an exhaust that has a nice note and volume all the time, rather than having something loud and raspy when open and muffeled to high hello when closed. I'm sure there'd be a thread on varex products on here somewhere.