The twin loop design is just a sound-tuning feature. It uses a tuned length of pipe to cancel a specific range of sound frequencies. It's a nice way to help reduce and tune the sound without increasing backpressure. In much the same way as the Flowmaster internal chamber designs.
In my personal experience, the performance difference between a premium, well designed, twin-loop, flowmaster, or regular twin or triple chamber turbo design is pretty much negligible. They do all have different sound characteristics though. Straight-through glass-packs have substantially less back pressure but are extremely loud (and usually illegal).
Read texan's "The Myth of Backpressure":
http://www.automotiveforums.com/vbulletin/t13199.html