Ah yes, outdated info. Someone please update that thread!
If you own the horse, you can see what stat's it gets for each heirloom level if you click 'retire' on the c-rpg.net site.
Here's the heirloom stats for the three horses i own besides my steppe horsey:
Arabian Warhorse
hit points: 90
body armor: 10
difficulty: 4
speed: 43
maneuver: 50
charge: 20
Well Bred Arabian Warhorse
hit points: 95
body armor: 10
difficulty: 4
speed: 43
maneuver: 51
charge: 21
Spirited Arabian Warhorse
hit points: 100
body armor: 10
difficulty: 4
speed: 44
maneuver: 52
charge: 22
Champion Arabian Warhorse
hit points: 105
body armor: 14
difficulty: 4
speed: 45
maneuver: 52
charge: 24
Courser
hit points: 100
body armor: 18
difficulty: 4
speed: 48
maneuver: 42
charge: 24
Well Bred Courser
hit points: 105
body armor: 18
difficulty: 4
speed: 48
maneuver: 43
charge: 25
Spirited Courser
hit points: 110
body armor: 18
difficulty: 4
speed: 49
maneuver: 44
charge: 26
Champion Courser
hit points: 115
body armor: 22
difficulty: 4
speed: 50
maneuver: 44
charge: 28
Cataphract Horse
hit points: 120
body armor: 47
difficulty: 5
speed: 38
maneuver: 40
charge: 38
Well Bred Cataphract Horse
hit points: 125
body armor: 47
difficulty: 5
speed: 38
maneuver: 41
charge: 39
Spirited Cataphract Horse
hit points: 130
body armor: 47
difficulty: 5
speed: 39
maneuver: 42
charge: 40
Champion Cataphract Horse
hit points: 135
body armor: 51
difficulty: 5
speed: 40
maneuver: 42
charge: 42
And a champion Steppe horse has:
hit points: 100
body armor: 18
difficulty: 3
speed: 40
maneuver: 51
charge: 18