From my perspective adding warehouse limits will break current economy look.
If you are comparing Settlers 3 and Settlers 2 (I will use S3 &S2 names), there are significant differences:
1. Roads/ transportation system
In S2 you have roads (the same here in Widelands), but in S3 you don't have them. Instead of this in S3 you have limited number of carriers.
2. Buildings wares
In all S2, S3 and Wide you have some kind of "warehouse" inside each production building, but only in S3 you have mentioned "warehouse" in its output (8 wares). In Wide and S2 you have some kind of "warehouse" on output, but it is very limited (S2 - 1 ware, Wide - didn't care, but probably more?) and you can't find direct data about it.
Since economy of S3 is based on pure carriers and "small" warehouses near each building, warehouses in S3 are not needed. I used them only when I needed to store gold and crystals, but only in some cases.
In S2 and Wide you have infinite limit of wares for warehouses and it is almost costless.
4. Interesting cases for those economies
In every game I tried to do some tricks to make my game better looking:
a) S2 water & wells
The transportation system was getting jammed easily. Especially with water. In Wide you have less productivity from the well, so it can't be easily jammed with water buckets. But in S2 I use a concept of one-way-away well and water prohibition. Every warehouse had "don't store water" and every well wasn't directly connected to any road. You had special short (usually 2-units long) dead end with a building at the end of it. After some time all wells produced 8 units of water and nothing more. If the water was needed by any other building (f.e. bakery), nearest bucket was called and usually it was nearest well. It was very good practice, especially on big maps.
b) S3 gold & crystals
Since storages were highly expensive (territory, not building cost), I tried to find alternative for them. And I have succeeded. Easiest situation: 2 or more islands. On the first island you produce gold ingots and send to the other one (remember to conquer it!) by a transport ships. If you don't have second island, make it on land. How? Send pioneers to a distance place, separate it from your territory and enemies (conquer everything around it) and then you have your "island". Use donkeys instead of ships. Sending anything to this place will be more useful since you don't have to build any storage house, but you are storing something. Also wares density is much higher than in standard building.
5. My idea how to build working economy in S2/Wide economy with S3 warehouse limits
Just stick to S3 solution: every building has to have it's own small warehouse. This warehouse is building's flag (as it was done in S2 water trick). So the transportation system will be much more complex, but still working. When it will be possible to do? Only on big maps, where you have almost limitless space.
Of course it is not the only solution. It is my first solution for such situation.
6. Impact on my strategy
I have wrote a paper how my strategy works. It is based on "bottleneck" idea: you produce more resources than it is needed to fulfil all economy needs. That is working strategy, but not best one. Best (known) one is based (as you mentioned above) on producing as much resources as are needed. So in general, you should have empty warehouses. But remember that in the late game you have plenty of building resources (wood & stone), regardless of strategy. And those resources can't stay at one place on roads. They have to be moved to warehouses.
Also some maps are designed that you have to cut trees (elven forest) or cut stones (the Nile) to expand. You have plenty of building resources and if you don't cut them, you have lack of useful space. So adding a warehouses limits means redesigning some maps and retesting hundred of cases.
7. conclusion - my opinion
I think that this idea should not be implemented and will not improve Widelands at this point. It would change the game mechanics in major way and can break some parts of it. So I am against it.
If the community says "yes" to this idea, please consider proper preparing for this feature by teaching AI how to solve problems of infinite wares and probably making tutorial for basic economy settings. Maybe build 23 will be proper milestone for the idea?