So the benefits for the Steed Stone is: +100 carry weight and, no movement penalty from armor. What does that mean exactly? Does that mean it's just as easy to sneak with heavy armor than with light armor or does that mean running in heavy armor is the same as light to no armor?

i use the steed stone for the extra 100 carry weight and i have been wondering this as well. i am currently wearing half orc and half ebony heavy armor and i am very noisy.

It means you run as if you were naked with any armor, and all armor you're WEARING doesn't count against you in encumbrance points.

So not only do I get the extra 100 carry but all armor I am wearing doesn't count against my carry load too?