So, I got my white horse for 100g. Spent a fair amount of time riding around though I don't know why - Diamond is only MARGINALLY faster than my normal run speed in heavy armor. Folks in light armor can easily out-run my faithful steed; not sure about medium.
I never tried feeding Diamond - perhaps that was my issue. But my goodness - she's one slow mare. So there's no mobility advantage from the white imperial horse. But it does look cool. That's about it.