Re: ford 2008 f250 performance
Allowing more air out of the engine increases in noise. If you want power, you need to allow the air to move freely. Mandrel bent tubes and a free flowing muffler is the way to go. You don't need to increase the diameter of the exhaust. If you do, then it would be best to upgrade the headers and Y pipe while you're at it. Otherwise, you'll be wasting your money on an exhaust setup. A cold air intake setup, muffler (exhaust is up to you). Depending on the state you live in, you might be able to remove the cat converter. If you can, then a tuner will make the engine light go away as the O2 sensors will trigger an engine code.
Going with a large diameter exhaust without the intake flow (supercharger for instance), can actually hamper performance as you're removing too much backpressure. You might gain a few HP, but you'll lose significant torque. Keep the same diameter unless you plan on going 'all out' with high end intake mods.
Some things that are out there, are better cam profiles for better breathing. I'm not overly familiar with the 5.4L, but if you don't use the truck, as a truck (towing, etc), then some F150 aftermarket parts will work. If you really want to piece things together to squeeze some power out of the truck you'll need a full custom tune or you can damage the engine. Either way you'll be looking at ~$1500-2000 in parts (you cover the labour) to get 25-30 useable HP. When you look at advertised HP on any mod, you cannot add up the HP in the hope of thinking 5+5+5+5=20HP. In reality once those mods are connected to the same engine you might only see a 50% of the advertised gain. A custom tuner can help you but depending on what you want (mid range/low end/high end) power wise, your best bet is to consult a local performance shop.
If you don't drive it much on the highway, and mostly in town, I would just swap out the muffler (no pipes), add a cold air intake (CAI) so the engine can breathe better. This way you don't need a tune, and you can throw in some 4.56 gears to replace your 3.73 gears. If you have 4.10's already going to 4.56 won't really be worth while. You will notice a difference in off the line performance if you have 3.73 gears. Driving in town should also increase mileage. If you have a 4x4, you'll need to get the gears changed in both the front and rear axle. If it's 2wd only, then you can get the rear gears swapped for roughly $750 depending on the shop.
There might be more things you can do power wise in terms of cam profiles to the 4V 5.4 versus the 2V 5.4, but you'll need to do further research on your end. The best bet is to consult the Mustang/F150 guys that deal with that engine on a regular basis.
The thing you need to consider if you plan on selling the truck before it becomes completely worthless is resale value. Taking a truck and making the engine more in line with a lighter vehicle (taking away the capability) can affect long term durability as well.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!