Yeah you need to be on a fast or agile horse for lancer cav to be worth it. If you use a regular lance, then a destrier is fast enough to still do good damage. Any armored horses and I'd recommend not using a lance.
I love using MW Lance on Champ Courser, a heavy lance would be good on a courser or arabian as well
I wouldn't recommend it either. Heavy horse + lance is a pretty bad combination, specially with only 5 riding.
But after 2 years playing with courser and arabian warhorse (both with shield and heavy lance) I wanted to try something new.
So since about 6 months I've been playing with strength builds using destrier or above without shield.
I've been riding the large warhorse and the mamluk for some months now. Both +3.
Yesterday I bought a +3 destrier. Damn it died easily (from ranged)... I was a bit chocked.
A shield really is a good thing to have on a lighter horse, that's for sure.
With a horse such as the mamluk, you can take lots of ranged hits if you take it slow and it rarely dies from a shot in the head unless there is a speed bonus.
By the way, I'm using a 27/15 build.