For a keystone, Phase Rush can't really compare to other keystones, in that, the movement speed buff is the same at level 18 to the current Stormraider's Surge. Phase Rush is just a weaker Stormraider's that can proc a little easier for some champs.
-The 75% slow resistance is for melee only whereas Stormraider's just provides the buff to everybody
-The cooldown is 5 secs higher for Phase Rush
-Movement speed buff is lower (equal at level 18)
Phase Rush should go back to the original 30-60% buff, or at least get a sizeable buff to get it to the level of other keystones like grasp.