I'm positive that this has been brought up before but I'm going to bring it up again anyway.
As a dedicated horseman, there is no reason -- besides upkeep -- to not wear heavy armor.
An infantryman loses speed when he wears heavy armor. What does a horseman lose? Surely a slight amount attack speed, but any skilled horseman knows not put himself in a situation where attack speed is a factor in his survival.
Say two players mount two coursers. The first player is in full plate, the second player is naked. They race a short distance. Who should win? Did someone say the naked guy? Wrong, they tie. A few seconds later, two crossbowmen fire their crossbows into the players mounted on the coursers. The naked guy dies and the armored guy lives with two thirds of his HP to spare. So why should I be light cavalry?
Well, if your horse dies, you're slightly faster. But I'm a dedicated horseman, I don't give a damn about how quick I am on foot!
You don't have to pay as much for upkeep. But I made the conscience choice to be light cavalry, not because I'm poor!
In history, what did light cavalry do? Yes, a large amount of their appeal was in communications and reconnaissance, but they had a number of battlefield advantages over heavy cavalry. The obvious is enhanced mobility. During the Scottish Wars of Independence, light cavalry were the deciding factor in many battles. Take the Battle of Bannockburn where five-hundred light cavalry effectively caused the whole of the English army to retreat by dispersing the English longbowmen. This leads to my suggestion: the weight of your gear should effect the mobility and acceleration of your horse. Notice I said "acceleration" instead of "speed." Heavy cavalry typically didn't weave through the battlefield as we see in cRPG. They are meant to be as shock-and-awe chargers, and unfortunately that is not represented in this mod.
I think we can all gain from the implementation of this suggestion. No longer would plated chargers with Milanese knights on their backs do 180o turns in two seconds, and no longer would every horseman that isn't a horse archer feel the need to wear full plate.