I know this will be hard to believe because it goes against generally accepted hot rodding practices but exhaust mods produce little or no gains in performance in the L. Ford actually did their homework on this system.
It has been documented time and time again over the years through dyno runs and track times on f150online that exhaust mods will, at best, yield 5 - 8 hp. A full exhaust replacement with headers are worth maybe 10 - 12 hp (long tubes exempted).
The timing of your question is kind of ironic. I just put on the Bassani cat pipe (2 high flow cats instead of 4) and Borla cat back from the 99 on the 01. When I bought them for the 99 I had no resources to learn about what difference certain mods made and never really tested the difference between stock and aftermarket. On the 01 though I am trying to document changes I make better. What I found was that I gained less than a tenth in et and less than 1 mph in the quarter mile which is typical from what I have read.
Now, on the upside, it sounds much, much better and, IMO, how the truck should have sounded from the factory. If I had it to do over again, I would have just bought the cat back system for the sound effects and spent the money for the pipes elsewhere. Here are a couple of links to listen to some systems.
http://www.borla.com/automotive/?car...366.51.195.211
http://www.geocities.com/whitetoast...l?1011835762130
JC Dyno Run
Bassani is the only company I know of that makes cat pipes for the L. Bassani (the most popular), Borla, Magnaflow, and Flowmaster (least popular) are some muffler systems available for the L. The tuners I listed for you previously can hook you up with most of these. Houston Performance, 281.893.6080 was the cheapest dealer for Bassani at the time I bought my pipes.
Hope it helps.