WPF does affect the damage you do with a couched lance (as does power strike), but the great lance is so strong that I wouldn't worry about putting points into polearm WPF if you're only using the great lance and no other polearms on foot.
I'd suggest getting 2 shield skill minimum (or 4 if you have the points for it) because you won't be able to cancel your couched attack without a shield (so you're stuck holding it for 8 seconds and have to wait another 8 for it to recover). When I use the great lance, there's lots of times I start the couch and 2 seconds later realize I'm not gonna be in line to hit my target and cancel it, then I only have to wait 2 seconds for it to reset, rather than having had to wait another 6 before it finishes and then 8 for the cool down.
There's some decent 2 skill shields you can use on horseback that would be good for projectiles, allow you to cancel your couch and allow you to take a couched lance and still keep on riding.
