Depends on how much you put in. The more packing you put in the more backpressure there will be and that means a power loss. I highly reccomend you take a look at this thread before you add more packing, it lists ways to reduce the amount of noise your engine makes.
I did everything FB said in his articles. Blue collar Bob also put out a good buch of suggestions if you search.
Packing (I did) It worked great. When all was said and done, My bike is 30%-40% quieter. Sure I sacrificed some speed but it's worth not getting attention!
The one other thing I did is turn the idle done to the point where the bike barely runs! I have to adjust it everytime I get on it, but it's worth it!
in the old days they sold some kind of packing at auto stores
think it was made from fiberglass
a fast trip to the local muffler shop should give some information
many such as my grandfather could not afford new mufflers
MM