I will answer the normal first question first (imagine that). The overload leafs weigh 25# apiece, as they are very thick steel. Taking them out saves 50# (or just under if you use spacers to reclaim the space, see below) of unsprung, rear, and gross weight, which is substantial, especially on some the Bs that are low in cargo capacity like a Roadtrek 210.
Nearly all the 2003+ Chevy class Bs sit right on the rear overload leafs, especially Roadtreks. The overload leaf is about 3/4" thick and very high spring rate, causing harshness and "pitching" on bumps of various sizes.
The most common thing done to address the issue is to add airbags in the rear, often with a spring change in the front to stay close to level (we did this over 5 years ago). A normal airbag install will get you 1/8" or so off the overload, but to get more you have to go pretty high on airbag pressure. Being that close will allow the overload leaf to still contact the springpack on moderate bumps, but will smooth out the small bumps. I had, a while ago, reached the overload to about 3/8" gap, which made it even better, but still contacted on bigger bumps with a "slap". The overload couldn't be moved further away as it was needed to pick up the weight if an airbag failed (the bags have a hard stop in them that is not designed to be driven on).
Airlift now has airbags that have a soft bump stop internally instead of the hardstop, and are now OK to drive on if failed or deflated. This makes the only function of the overload leaf go away, so they aren't needed for anything.
I decided to see how things would work out with the newer style bags and no overload leafs in place. The bag change is pretty easy at about 20 minutes per side. The removal of the overload leafs is more work at about an hour per side. The easiest way to do it is to remove the front eye bolt and rear shackle bolt and wrangle the entire spring to the rear without actually taking it out, as there is lots of stuff in the way, at least on our Roadtrek. You can get the spring just far enough back that the springpack center bolt can be removed. Once the center bolt is out, you can just slide the overload leaf out and remove it.
I had trouble deciding whether or not to cut out the center section of the overload leaf and reinstall it as a spacer, so the u-bolts and center bolt could be just as they were stock, but finally decided to do one side with the spacer and one without. Our Roadtrek has always required more pressure in the driver side airbag, which appears to be pretty common. The difference in height without the extra air pressure is just about the thickness of the overload leaf center area, so I put it in the driver side to lift it by that much, as it works just like a lift block. Passenger side no spacer, so I made some spacers for the u-bolt nut side and for the center bolt.
Note the pix were taken before the shackle and centerbolt spacers were in place.
Doing the shim on the driver side seems to have been about right, as we sit very close to perfect level with equal pressure in both airbags.
Of concern to me was if the internal bumpstop would give similar issues to the overload leafs on larger bumps, so I wanted to try to determine when they would be contacted. Before I put them in, I collapsed them in the bench vise to try to determine when the bumpstop contacted and how firm it was. The old hard stop was at 2" of bag height, and all air support if above that. With the new bags, full height is about 5.25", the bumpstop feel like it first hits at about 4", and it goes essentially solid at 2". These heights would translate very closely to what would happen with the overload leafs in the stock position at the same van heights, so my concerns remained. The bumpstops did seem relatively easy to compress in the vise however, so they did seem to have a substantially lower spring rate than the overload leafs.
In place, with 60# in the airbags, they measure at right about 3.5", so lightly on the bumpstops. This van height is very similar to where the van sat with the old bags, and not touching the overload leafs, so the bumpstops aren't carrying much weight at that point, it appears.
We went for about a 60 mile ride and the rear did seem to be noticeably quieter and smoother, into big bumps especially. I assume this is because the transition onto the bump stops in the bags is less progressive and lower rate than the overloads were. We will know more when we get longer and more varied trips on them.