I tow a 6300lb travel trailer that puts about 1000 lb on the tongue of my 2017 EL. I use a 1000lb WDH. I had the SumoSprings on my stock rear springs/shocks, but it was still too bouncy. I wanted something adjustable (tow vs. street vs. off-road) so I went with the Rancho Quick Lift Loaded struts in the rear (p/n RS999926 + RS999941) and added SumoSprings to those too. Unfortunately, I had to buy another set of SumoSprings as the Rancho spring coils were closer together (1.25" vs 1.68" for stock springs). Towing is much nicer in all but the wallowy-est roads (urban Los Angeles freeways). Towing on wide open roads/countrysides, curvy mountain roads, or long desert highways are all much improved.
The ride with the SumoSprings (and Ranchos) is not rougher. You can compensate for harshness (noise) with tire pressures - I typically over-inflate my LR E truck tires to 60+ psi for better handling; 45-ish yields a much plusher and quieter ride. But "roughness" (over-spring) is not present with the stiffer Ranchos and SumoSprings unless the Rancho shocks are cranked up to 9/9 which I only use for towing. It could be that I'm biased towards sporty car suspension (stiff unibody + taught suspension). On the street, I keep the shocks at 7/9. For off-road anywhere between 3/9 to 6/9, depending on the terrain and the tires I'm running.
At this point I only wish I had gone with the Ranchos all around, but I had already invested in Bilstein 6112's which I now regret (too underdamped for the Expedition on undulating highways and roads with dips).
BTW, I do also have a upgraded rear sway bar (Hellwig) which helps with handling and lateral stability (crosswinds) when towing.
I do wish there were a King or Fox kit available for the rear of our Expeditions. I've only seen F-150 kits that we could use for our front suspension.
