I don't always 1 hit kill with the great lance on my champion courser, so the damage certainly will make a difference (on hilly terrain or if your target is running in the same direction as you). Also power strike and WPF increase the damage of couched lances.
I personally wouldn't suggest using a great lance without a shield (because you can't block anything and you can't always steer 100% away from enemy cavalry, especially if they have numbers).
15/21 is a good build for 1h/shield/polearm/cavalry. If you were going to go just polearm/2h then I would suggest going 18/21.
Here would be my suggested builds (using the arabian warhorse):
Level 30:
Strength: 15
Agility: 21
Hit points: 50
Converted: 2
Power Strike: 5
Shield: 4
Athletics: 7
Riding: 7
Weapon Master: 6
One Handed: 131
Polearm: 105
Level 30 (4 420 577 xp)
Strength: 18
Agility: 21
Hit points: 53
Converted: 8
Power Strike: 5
Athletics: 7
Riding: 7
Weapon Master: 4
Two Handed: 124
Polearm: 80
For the 2nd build, I'd probably give up even more polearm WPF to get more 2h WPF. Polearm WPF does increase the damage some of the couched lance, but not enough to be useful compared to having higher swing speed with your 2h. Hell even on the first build I'd probably drop the polearm WPF down into the 80s to get more 1h WPF. But again, that's only if you are never going to be using any other polearm besides the great lance. If you plan on using thrusting (or 4D) polearms, then I'd try to have about even WPF for either 1h or 2h.
