I agree with you Tom, "Completely Agree". If I am adding springs to my E350, I should have gotten an E450. If I was getting that new 2350 stretched version called a 2450, I'd surely pay the extra moiney for an E450 chassis.
As I mentioned before, our 2007 E350 does have a little rear-end sag. Just moments ago I placed my 4 foot level on the vinyl flooring and confirmed our floor has a minor slope down to the rear. This considering that our rig is loaded up minus fresh water, food and clothing. The slope will surely get worse when loaded for a trip. I never considered the sag to be of concern as we don't feel the slope, nor do we scrape the street when exiting gas stations and such. It just doesn't look quite right. You can see what I mean when closely examining this picture, taken during a trip.

We do have a unique matter concerning our garage. If we raised the rig that 2", our a/c cover would not clear the door frame. That is why I consider the Firestone air bags which would allow me to lower the rig when back at home.
I'd like all your opinions looking at my picture. Do you feel my sag is worth doing something about? It is more of a "mental thing" to me.....so I think. I suppose if we had a slide out, the condition would be more severe.