Self Leveling Really Necessary for Towing 5,000lbs?

[F150 Duke]

All,

I am looking at potentially getting the 35d (50i) is just hard to justify the extra $5k. As explained in other threads I'll be towing a 5,000lb boat and trailer this summer about once a week.

I was going to include the 3rd row seats to get the self leveling suspension to help keep the rear from sagging when the boat trailer is on the hitch. Does anyone know if it'll really counter that much weight or if that 5,000lbs (500lb tongue weight) will really cause the rear to sag? I'd really like to save that $1,700. But then again the boat trailer caused the back of my 2005 F150 to sag 1.5 inches so I have to imagine the X5 would be noticeably low in the back and that would impact ride quality.

Thanks!