Just bought a 2006 F-150 XLT 5.4L XCAB with a 78" bed. I'm looking to put a different exhaust on it that will give me more horsepower and a louder sound, either dual or side exit. I would like to spend around $500 to $600 on it. Any recommendations would be great. I was also wondering how adding a new exhaust affects gas milage. One last thing, I spoke with a few vendors were I live and they said that I kit for a 06 would not be out until next year, but yet I can find them online. Does anyone know why they might be saying that?

 

Maybe your exhaust shops are looking at dated catalogues?? Regardless, any decent exhaust shop should be able to take whatever muffler you want and mount it however you want.
As for added hp, Magnaflow claims hard numbers gained with their setup. I had a Magnaflow SI/SO on my '05 and ended up ditching it for straight-pipes. The Magnaflow had too much of a 'buzzing' tone for me. Sounded real drone-y, if you know what I mean. I was just looking for the traditional V8 lope, nice and mellow, with a little rumble to it.
Either way, my gas mileage hasn't really shown one lick of difference between the stock exhaust versus the Magnaflow, nor the straight-pipes. Or, maybe I've just been in the pedal more...;-)