I love the Courser's speed.
But remember that speed is allways working against you aswell!
Any hit you give OR take will get the same speed bonus boost.
Take archer that shoot you (behind some rock of course) now that arrow if it hits you gets your horsies speed bonus.
Courser can easily get 200% to 300% damage speed bonuses.
That Khergit bows 24 cut and Khergit arrows 7 pierce damage are now: 48 - 72 cut damage + 14 - 21 pierce damage = insta kill what ever it hits, be it the horse or the rider.
If you use Heavy Lance thats 27 pierce damage at full gallop those 200% or 300% speed bonus are now: 54 - 81 pierce damage = insta kill to all but heavy inf with lots of HP.
Now how many times in a round you get to attack a player that does not know how to down block or does not carry a shield or is otherwise clueless? (aka does not take cover in hills, buildings, etc.)
And, how many arrows, bolts or throwing weapons you think is gonno get fired at your horse in one round?
That Sarranid horse with less speed but more maneuver to dodge those arrows suddenly starts to sound like a good alternative.
Witch ever horse you choose, been cavalry is deadly (to both rider and victim) but ever so fun.