Larger tires can provide more 'balloon' to the overall ride, but will also add to the weight at each wheel, which increases the amount of Noise/Vibration Harshness (I think I'm using that term correctly)
Even with the airbag suspension on it's lowest setting, and my adjustable shocks turned down, the biggest difference in my ride comes from adjusting tire pressure. Everything else just seems to adjust the amount of pitch and porpoising that the van does.
Now, NVH can be a tricky thing, especially in a van with two fridges, non fabric blinds, a sink with dog toys, and all the various other things we have in the van. Just the sound of everything rattling around magnifies any bumps you may actually be feeling a lot.
My ride is relatively smooth, but all those small vibrations make everything in the van sound much worse than it actually is, and wears on the occupants as badly as anything else.
I made a HUGE improvement in this aspect by weather stripping all of my cabinets and drawers, as well as making sure my fridges are always full, or have tension bars in place to hold the contents still.
Also, I adjusted the tension on a few of my blinds so they wouldn't rattle against the window so much.
These steps actually went a lot further for me than airing down the tires to less than optimal air pressure. I still keep my rear tires at 70 and fronts at 65 for daily driving, but will air down to 45 for washboards.
It compares favorably to my memory of driving a smb 4x4 setup 9 months before I got my van, but that could very much be rose colored lenses.
When I get off the dirt though, and before I air back up, the ride is a little like an older caddy
I will say this is the thing I look forward to most whenever a get together can happen that I can attend. I would very much like to compare my ride side by side with some of the other vans, because I get quite a few questions about it. Also it would be nice to compare turning radii, stopping distances, and maybe even a little articulation, but now I'm asking for a mini-olympics. Maybe at the next forum rally?
I will say that the airbags are super nice for adjusting the ride when we get extra passengers, primitive leveling when camping, and it is nice to be able to adjust the spring rate on the fly if I get on a road that makes the van want to bounce a lot. (yolo causeway, for example)
I still think most of the suspension work is being done by the springs, and I could even totally lose pressure on the bags and it would ride safely. (this was a design target from S&K). They added a leaf to the rears and those front springs are pretty burly. In theory I could reduce my spring rate by going with smaller springs and removing that one leaf, but then if I had a failure on my air springs I would be a sad panda.