are there gains to be had? and does it require pcm flashing? also are headers a easy install?

 

Are you running a GT or V6? The stock exhaust manifolds on the GT are pretty good. If you are going to replace them, replace them with long tube headers only. Shorty headers won't make enough of a difference to justify the cost. Typically from what I've seen shorty headers alone on the GT will only get you 4 - 7 hp. For several hundred dollars that just doesn't make sense. Long tube headers on the other hand will be good for about 20 hp IF you also remove the cats. If you have to leave the cats in you will see an increase of about 12 - 16 hp. This is assuming the remainder of the exhaust system is left intact. Also be aware that Ford did an oustanding job of the rest of the exhaust system and the aftermarket systems offer little to no gain in performance. If you want a different sound that's another story. The aftermarket setups might offer what you are looking for it all you're after is a different sound.
You will also need to retune the computer for changes like this. Get a Diablosport Predator or an Xcal unit.
Under drive pulleys will help a little. Usually around 7 - 10 hp. Easy to install. No computer changes required. Just be aware that some of these under drive pulley sets use a modified balancer. Since engine balance is very critical, the design of the balancer also is critical. I would suggest buying ONLY from a reputable manufacturer like Steeda. Sean Hyland Motorsports has a very good article on the importance of balancer design on the Ford MOD engines. I'd suggest visiting thier web site and reading that.
Best of luck!