Take out the inline muffler(resonator) and change stock muffler to a performance muffler for a few extra horsepower(5-10hp approx). Your exhaust will also sound a lot better. Use a K & N cone filter. It will give around 2-3 hp increase.Buy a performace chip(3-9hp approx). These are about the cheapest you can do.
First of all, if you take out the Resonator it will give you a very loud sound since the resonator its like a pre-muffler. Although, you said if someone takes out the resonator that it will give you 3-5hp which is very very incorrect information you are giving. According to this, the resonator will gain much air flowing going to the mufflers and probably someone will notice a gain but not for much. The K&N panel filter will give you something so it will sound very nice at WOT and if you take out the Intake Air Silence from the fender which will sounds very nice too.
Remove or gut the cats, that will be the bigges exhaust mod. 5+HP. I gained 2-2.5 tenths in the quarter mile. And get a custom burned chip...bout $300. Thoes two will net the best bang for buck and you will DEFINATLY notice it.
Please use the search function as it will help you fint TONZ of cool mods.
Removing the exhaust resonator will not do ANYTHING but make your exhaust sound different. The resonator is nothing more than a larger diameter pipe, cut to a certain length to tune the pitch of the exhaust. There's nothing inside of it, and removing it will do nothing but waste your time. If you want good, cheap mods, do the stuff above.
A K&N panel filter is definitely worth it though. You're not going to notice any real difference in speed, but you'll notice that the intake sounds better and the K&N filters out ALOT more dirt than a traditional paper filter will.
The inline resonator isn't just a staight pipe with nothing in it, its a premuffler to quit down the interior noise of the exhaust. It will make a your exhaust a bit more free flowing without it and I said with a performance muffler added on will give 5-10 more horsepower, not just taking out the resonator. The sound you get depends on the performance muffler you put on.Maybe with just the resonator off and stock muffler on it won't sound good but with the right performance muffler it sounds great.I've heard it on a friends Taurus and it sounds very deep and high perf, not raspy at all.
I really recommend what Brad said by taking out the cats if they do not check emission in your area. I had high flow cats and I noticed something but not a huge improvement, plus at that time I had 2 Flowmaster and it sounded really sweet in my Vulcan. I got rid of those flowmaster because I had some kind of problems with the police. He said my muffler was very very loud so I had to do it. Next a few days ago I just went to the junkyard I saw a G.3 Sable and I cut the Y pipe and the with the 2 mufflers in its place. Well to make it short I just took out the 2 cats and damn my car its a kinda of faster than it was before. Therefore, taking out the cats and putting in a chip thats a cheap mod you might consider for the buck.
Resonators in oem use tend to be packed with steel wool and function to both reduce noise level and tune the note. While they may look like just straight pipe, there is an inner perforated tube that holds the fiber against the outer shell.