From my research on this subject, removing the muffler is going to have a negative effect on the performance of the truck in most of the range that you are going to use it. By removing the muffler, you are dropping the back pressure which is great for releasing power. So, yes, you can get a lot more power out of the engine by doing something as simple as getting rid of the muffler. Obviously, this is going to result in a louder exhaust note.
The downside to this is that motors require back pressure to develop low end torque. So, this mod is great for when you are opening up the throttle and hitting 5,000 RPM. But, if you want something to get off the line with at say 1500 RPM, this will kill that part of the power curve. Installing something like an Edge programmer may help restore some of this power loss, but it won't replace it all. You may even see a slight drop in your mileage as the motor still can't develop enough back pressure up at highway cruising speeds (more of an issue with non-Cali emission vehicles).
If you stick with the stock 2.25" single exhaust pipe, this will also offset this to a degree (provides a certain amount of restriction due to the limited pipe size). But, if you open up the exhaust by going to say a true dual 2.5" setup or a single 3" setup, you are again asking for problems.
There is no "perfect" exhaust system. It is a balance between compromises. So, how do you use your truck? Me, I would never consider this sort of mod as I want all the low end grunt I can get. Hence why I have gone with a 70 series muffler but a true dual setup. This allows me to pull Volvos sideways on asphault (should have seen the kids face when I did that).
