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Somebody said it correctly above. Dyno results can go +-10hp just based on ambient temperature and the calibration of the dyno (most stink).
On my '04 FX4 had the Airraid, Magnaflow, and Superchip. Didn't bother with dyno. Hauled the race car to the same tracks and could accelerate up the steep hills after the install. Drove with a guy in a Titan that used to pull me on any stop, any hill. After the mods, the situation reversed-I was lovin' that!
For the HP based racing classes, you can plus or minus yourself 10-15hp just based on the time of day and the fan/speed simulation.
All that said, if it makes you happy to put any/all/none on your truck, it's all about the fun.