rayman009 said:
does adding just a muffler add any HP? or is just for show?
I think you have to look at it in reverse. The stock mufflers exist solely to suppress sound. The stock mufflers use chambers that rely on deflection to create noise canceling frequencies. An unwanted side effect is that some power is lost in this process of sound reduction. The TL also relies on b-pipe resonators to cancel out sound. It all has to do with chambers of air pressure to create noise canceling frequencies by deflection.
The advantage of many aftermarket performance mufflers is that they suppress sound strictly by absorption instead of power robbing deflection. Hence, the term straight-through design. You'll see in the performance muffler that there is a perforated tube that extends from one end of the muffler to the other. The performance muffler has sound absorbing material outside of the perforated tube within the muffler canister, usually fiberglass or steel wool mesh. The idea is to minimize backpressure and maximize performance efficiency.
This is a cutaway of the HKS Drager exhaust.
Now how well a performance aftermarket muffler works on a TL is circumspect because of the b-pipe resonators and, i believe, a relatively small piping diameter that the car comes with. In other words, if you want measurable power gains, I would go with a complete cat-back exhaust.