I know about the 800 lbs of payload added. But what about mechanical add-ons?
I thing better shocks, extra blade on rear suspension...what about brake? Are they the same?

 

Thicker frame, axle wall thickness, springs, wheels, tires.

 

Here are the contents of both Payload Packages

 

The thing that stinks about the Ford documentation above is that Ford randomly leaves out important distinctions. And the thing that makes this question hard to answer is "compared to what?". So I'll try to include a few comparisons below. If you don't believe any of what I have below, talk to your local Ford parts department and get specs for replacement parts. The HDPP referenced is 2018.

Frame: HDPP gets the 0.11" frame stock compared to 0.10" for heavier non-HDPP trucks and 0.087" for lighter F150's (most of the 2.7EB trucks)
Front coil: stiffer than any non-HDPP truck but the same front axle rating (3800 pounds) due to some other limitation
Rear leaves: 3+1 spring pack rated at 4800 pounds instead of the 2+1 for all other weigh ratings (4050 [2018 MaxTow], 3800 [most 3.5EB/5.0 trucks], 3300 [most 2.7 trucks])
Rear axle: 9.75HD axle (9.75B on axle sticker) rated at 4800 pounds and with the electronic locking 3.73 gears. The 9.75 on most 3.5EB (including Max tow) is a lighter duty variant and the 8.8 on many non 3.5EB trucks is lighter yet.
Shocks: Heavier duty shocks than any non-HDPP F150
Wheels: Slightly heavier than non-HDPP wheels (rated at 2275 pounds)
Tires: Slightly heavier LT tires that have actual tread and are quite good in snow - unlike most the other OEM options.

The increased cooling and front sway bar all seem to be available with some of the tow packages so not unique to HDPP. I still need to check the part for the front sway bar to see if it is the same across the tow packages and HDPP

 

Or I could have just wrote "what he said" and added "shocks"

 

Regarding brakes - I believe all F150's have the same size brakes 13.7"/13.8" rotors - which, by the way, are larger rotors than a year 2000 F350 dually.

 

As far as your frame thickness numbers, that would depend on where you measure it, since each frame has varying thickness throughout as part of weight savings.

 

These measurements come from Ford themselves, not owners with $8 harbor freight calipers.

 

Ok. The frames are not uniform thickness throughout. The thickness varies on each frame. Where are they 0.11", 0.10" and 0.087"? And what are the other measurements for each frame?

 

I have not seen the other frame measurements published from Ford. I'm attaching the Ford document with the frame thickness. That's all the information I have.... I think the point is that if Ford made the frame stronger at one point, Ford also would make it stronger anywhere else that counts. These frames are engineered front to back to meet design specs, so I think the differences shown below (even though they are taken at the max cross section) are a good indicator of the overall differences in frame strength.