Eh, didn't know Nords had a bonus to light armor. I just switched to Dwarven armor. Does the bonus make enough difference to switch to a weaker armor?
The bonus just gives you a jumpstart in those categories. IE a Nord will start at level 20 in light armor instead of 15 (+5). It still maxes at 100. So no, there's not much difference. Also, there is an armor cap, I forget what the exact number is, but at a certain armor level, more armor doesn't offer more protection, so I think you can get light armor that has as much protection as heavy armor if you use potions and enchanting.
Damage reduction is capped at 80%. If you are wearing all four pieces of armor, this occurs at 567 displayed armor rating.