Does the steed stone boost sneaking?

Yes, it does help. I have personal experience with it, but no links to 'official' words on it.

The basis is that I wear heavy armor and sneak only on occasion, and noticed once I switched to the Steed, I was able to keep the 'running sneak' active around enemies much longer. My sneak skill is low 20s, and had not increased before I noticed. I do not sneak much, so when I do on that character I really notice the difference (I was actually removing all armor to try and sneak on him before getting the Steed stone).

As a side note, remember that weight is not the only thing that relates to sneaking. I have no perks on this character before OR after steed to assist with sneaking. But there are several available, as well as spells/potions.

Short answer:

Yes, having the Steed bonus will reduce, but not eliminate, armor sneak penalties.

Detection Value is made up of:

• Enemy Base Sneak +
• Sound generated by the player +
• Visibility of the player +
• Enemy Sneak Skill +
• Player Sneak Level Bonus

Sound is the sum of Movement Noise + Weapon Noise

Here's the equation for movement noise:

Movement Sound = ( fSneakEquippedWeightBase + fSneakEquippedWeightMult * armor weight)

• Movement Noise is modified by how much weight you have equipped and what type of weight you have equipped. 5 pounds of Heavy Armor makes more noise than 5 pounds of Light Armor. Since there is always a base noise value from having any weight equipped, removing armor weight using the Steed Stone will reduce, but not eliminate, total movement sound.

• Weapon Noise is created when you swing any weapon other than a dagger. The Steed stone does not eliminate these sounds.

Source: Bethsoft Creation Kit