In Vanilla, Sleek leg mods don't stack with each other. One legpiece with a Sleek mod is the same as two legpieces with Sleek mods.

This mod makes it so that two legpieces with Sleek mods give you a little more movement speed in Stealth. I'm pretty sure it's a bug because of the way the records are set in the base .esm.

Created with FO4Edit.

To Install:

Copy the .esp into your /Data/ folder and activate it with the manager of your choice.

Compatibility:

This mod edits the effect and not the item. Therefore it's compatible with any and all mods that alter items, including the various wonderful crafting mods, and it will Just Work(tm).

It will only conflict with any mods that alter the "movement speed while in Stealth" effect (i.e. mods that "+500% movement speed when stealthed!" or "bonus stealth speed even while not stealthed!"). However, to make those mods work, they would have to edit the same record I edited, so just use that and not this.

When in doubt, load this first and load theirs after.

Testing Details:

I did extensive scientific testing by placing a bunch of wooden floors to create a runway and then timing my runs back and forth using the beat in LP's Levitator as a stopwatch. It's such a great song. You should listen to it. Anyway, in vanilla:

Sprinter's legendary modifier (+10% SpeedMult) stacks properly, stealthed or not. (that is: movement 0x Sprinter's < 1x Sprinter's < 2x Sprinter's).
1x Sleek leg modifier stacks properly with 0x Sprinter's, 1x Sprinter's, and 2x Sprinter's.
2x Sleek leg modifier affects movement speed exactly the same as 1x. That is: movement 1x Sleek 0x Sprinter's = 2x Sleek 0x Sprinters. 1x Sleek 2x Sprinter's = 2x Sleek 2x Sprinter's.

Technical Details:

Effects are what you'd expect if you look at the enchArmorStealthMoveMod record, but I wanted to test it in-game just to make sure.

I think it's a bug because if it wasn't meant to stack, there would be no need to set two conditions on the record.

Vanilla records:
If Mod_StealthMove_AV = 5, then AbFortifyActorSpeedMult = 5
If Mod_StealthMove_AV = 10, then AbFortifyActorSpeedMult = 5

If it wasn't meant to stack, it should just be:
If Mod_StealthMove_AV >= 5, then AbFortifyActorSpeedMult = 5

This mod sets it so:
If Mod_StealthMove_AV = 5, then AbFortifyActorSpeedMult = 5
If Mod_StealthMove_AV = 10, then AbFortifyActorSpeedMult = 10