Realistically, what you'd be looking at is a difference of ~20 lbs (at max, and I can't help from feeling that this is rather generous) from the heavier to the more common loadouts for riders. For a 1300 lb destrier, thats not even 2% of its body weight. As I said before, you'll see an impact in endurance, not its max speed or acceleration. The only reason we can even have this conversation, is because armor weight in cRPG is unrealistic. So I'm not sure if that's where you're getting 70 lbs in armor, but that's nearly double what it would actually weigh.
Edit: As Jarold stated on the first page, the only horses that would be penalized by this, would be ones where it doesn't make sense to be using full plate armor to begin with.
Need a moderator to move this thread to the realism discussion section.