It will ride stiffer even w/ the proper pressure (not the PSI on the Door). It will handle better as it has less tire roll. The truck will also be slower as there is more rotating mass to move. P metric tires are much lighter then a LT. You don't need "E"s on a expedition, you'll be fine with some "C" or "D" ratings.
This is just me, but if you have a specific tire you're looking at that you can only get in an E load rating for the size you need, I'd just get it. I've put E's on several vehicles that didnt "need" 'em (2 Grand Cherokee's, a Bronco and a Blazer) and to be perfectly honest, I could never tell a hoot of difference in the ride. Maybe a smidge more noticeable feel over road cracks, but certainly nothing that equates to the comments most people make about how rough it'll make the ride.
I mean, if you put 80psi in 'em you'll get a rough ride -- but if you stay within the load limits of everything else (GVWR, GAWR, etc.) you'll never need that much pressure.
I know two guys who have E rated Goodyear ATS's on their Expy's and I've ridden in both and they ride exactly the same as mine.
And just because I know somebody will ask, I'll give you one example of why you might go with an E tire on a truck that doesnt "need" it. I had an '01 Grand Cherokee with a 2" lift and 17" wheels... I wanted a more agressive off-road tire because I did a LOT of 4-wheelin in the Colorado mountains. Well, the only tire I could find (other than no-name generic brands) with a tread design I liked, and that made a 17" tire that would fit inside my wheel wells and clear while turning and under extreme articulation was a Cooper Discoverer STT -- and in the paticular size I needed they only made an E.
I got an 07 Tundra with BFG AT T/A KOs. Stock tire is 275/65/18. The new one is 285/65/18. It's definitely a rougher ride - because I run 50/50 psi in there as opposed to some bull***** like 30/33 on the door. When I run them down at about 40 there's little difference. I like the stiff ride of 50/50 plus I can get max tire life out of them there.
If you want to sacrifice a little wear you can run them at door pressures but just buy a depth gauge and check it. Buddy of mine that works at Firestone said within 1.5 mm from center to outside is normal wear.
I put cooper discoverer ATR's on every one of my personal trucks and our fleet at work. we put about 15000 miles a year on each of our work trucks driving highway and gravel and I think these are the best tires you could get for the money. I only get E rated because we get into allot of rough gravel and were getting more flats with the lighter tires and with the ATR's the wife can't even tell a difference in road noise. we wind up replacing them about every 2 - 2.5 years. The only other tires I would consider are yokoham geolanders I hear allot of good things about these from the guys that run the oilfields up north.
Thanks for the discussion, I actually was looking at the Cooper ATR's for my Expy to replace the Continentals. I was thinking about the Cooper H/T's because I had a set of those before and most of my driving (90%) is highway.