I agree with the thought line of reducing the output down to .25/stable and bringing back the 2nd level stable (to .5), but put it deep in the tech tree.
Changing it to a Civ 5 style (capped output) for a single resource doesn't work, given that all other resources in the game are built up over time so we should keep to that style. It also opens up many other issues (lack of the resources/etc) that would need work, but for a single resource it doesn't make sense to go that way.
Other changes that can be made:
remove the ability to 'defend' for mounted units. That gives foot soldiers a boost vs. mounted as they'd be given the 'dig in' ability for better defense and dodging.
Increase the number of horses/unit required from 1 to 2. So a company would require 8 horses rather than 4. This is fairly historical as armies would need extra mounts due to them getting worn down in battle or from scouting missions.
The bonuses mounted units give should also be adjusted. Frankly, I can't see much reason to take a horse over a warg, given the removal of encumbrance. Even worse is the War Horse, as it's not really that much better.
I'd like to see the war horse get +2 defense/model, but drop 50 points on the bonus vs. ranged. It would also be nice for it to get an armour offset bonus as war horses were bred for carrying heavier troops. The Wargs +initiative bonus would need to be removed as it is currently the unit that can be used for plate armour the best as it directly counters the initiative penalties from armour.
In exchange, I'd give it more of a dodging bonus.
On top of that, mounts should be given an armour proficiency requirement maximum. Plate mail for Warhorses, Chain for horses and wargs then whatever else for the fantastical mounts.