The sleek mods each add 5 to the actor value Mod_StealthMove_AV [AVIF:00245BEB] and apply the Stealth Move enchantment (00245BDE), which in turn uses the actor value to fortify (add) 5 to the actor speed mult (which is usually at 1) to result in 1.05 (5 % increase). Its condition is "if Mod_StealthMove_AV equal to 5." It also fortifies (adds) 5 to the actor speed mult "if Mod_StealthMove_AV equal to 10."
Obviously, Mod_StealthMove_AV will only either equal to 5 or equal to 10, so you don't get the benefits of both effects in just one enchantment.
If the game allows an enchantment to get applied twice (I assume it does, going off of this answer here), then the effect will stack to 1.10 (10%).
After testing ingame I can confirm that the bonuses do indeed stack, and that the speedmult goes from 1 to 1.05 with one mod and then to 1.10 with two mods. Should have just tested first and left out the technical stuff.