I'm a little surprised it isn't a regular cab with the 2.7L V6 that gets the highest payload. But they manipulate the suspension components a fair bit between different models so it's not just going to come down to the weight of the engine. The Lightning weighs as much as a gasser F250 but still has a payload in the 1650 pound range; my guess (without doing any research to prove it, mind you) is that the Lightning has the highest GVWR of any F150. I think even the HDPP only gets an ICE F150 up to 7850 GVWR.

On a related note, we have some real candidates on the Lightning forums for being the modern "Danger Ranger" trucks -- turns out you can load a Lightning with well north of 2000 pounds and it still isn't squatting anywhere near the bump stops. Stiff suspension.