Storm Raiders give less movement speed late game.....
I decided to try this keystone out and realized that as you buy movement speed items, the 40% movespeed you gain from storm raider diminishes because of the movement speed cap in the game. This actually makes the keystone weaker as late game approaches.
If you have more than 415 movespeed then you get capped at 80% of anything over 415. The raw speed over 490 gets multiplied by 50%.
So, if your support buffs you with a movespeed spell, the movespeed from stormraider becomes useless.
The only thing you really gain is the slow resist which can be hard to use if you're not an assassin. Even if you are an assassin, you won't last long enough to make use of the slow resist.
All in all, storm raiders seems kinda underwhelming...