The M113 5.5L Na Performance Pack Includes.
ECU Tune ( via OBD2 cable from a laptop)
TCU Tune ( Gearbox computer, tuned unit supplied
- unplug old... plug in new unit )
Performance Spark Plugs, (16 plugs)
Performance Air Filters
This package will Gain around 15 - 20HP & 20TQ
At the wheels, maybe abit more.
To give you a reference. The 5.5 NA will make about 270hp at the wheels in the real world conditions.
So i d say you will get between 8% and 10% gain.