The actual HDPP (3.5 or 5.0 only) comes with a few additions over a 2.7PP or a Max Tow 3.5/5.0.
The HDPP is the easiest (and often only) route to get 3.73 gears with the 3.5 Ecoboost.
The HDPP mandates Max Tow, so all of those features are included. Trailer brake controller, rear receiver, bumper, 3/4-float 9.75 rear end, etc.
In addition to the Max Tow features, the HDPP ensures that the HDPP frame is installed in lieu of the LD or HD frames. The HDPP frame is the thickest and stiffest frame available.
The HDPP also installs the stiffest rate rear springs, which are not available in any other configuration, including Max Tow. They are 2+1 leaves similar to the lighter duty Max Tow (non-HDPP) springs, but are thicker and have higher stiffness/load capacity. HDPP always results in a RGAWR of 4800 lbs. Most non-HDPP models are rated to 4150 RGAWR or less, sometimes as low as 3400. (Certain Powerboost models are rated at 4350 due to the significant additional weight of the electric powertrain components, but the payload is still very low.)
The HDPP also mandates the use of one of the top two or three stiffness front springs, depending upon the cab/bed configuration. This also provides additional front end load support for the heavier frame, powertrain, etc. It also provides more FGAWR headroom for heavy front end accessories like grille guards, bumpers, plows, etc. The front end also usually bears a small amount of the bed weight as the 8-foot bed is centered forward of the rear axle. I believe it usually uprates FGAWR to 3750 lbs, but in the case of the regular cab it might be 3450. Diesel models might go a bit higher.
Finally, the HDPP provides the legal 7850 lbs GVWR that ensures that loading up to that amount is of minimal liability and a non-issue legally.
