The FICM tunes do not derive their power by advancing the timing like ECM-based tuners do. This advancing of the timing is what causes cylinder head pressures to increase.
The Atlas 40, Atlas 80, and Hercules tunes get their power by prolonging the burn. Basically, fuel is added to the fire already going as a result of the explosion in the cylinder. Testing performed by PHP confirmed that the tunes do not increase the pressure that the cylinders are under, but rather increase the amount of time that the cylinders are under pressure.
The result is increased exhaust gas temperatures with the Atlas 40, Atlas 80, and Hercules tunes. Exhaust gas temperatures do not differ from stock with the Eco tune.
A quick shout out goes to David Lott with Diesel Innovations for pointing out the common misconception that the 2003, 2004 (and some say even 2005) model years somehow have less clamping force on the cylinder heads due to the use of 18mm head bolts. Actually, motors produced prior to January 11, 2006 use 18mm locating dowel pins - the actual head bolt size and resultant clamping force is identical between these earlier motors and the ones following that came with 20mm locating dowel pins. The reason for the dowel pin size was in preparation for the creation of the new 6.4L motors.