If we're talking trash mobs, 5% is insignificant. You can overstate the effectiveness, but it doesn't change the fact that awareness and foresight, the two absolute weakest tank cd's, mitigate more damage in their respective durations than SF's Slow does in two minutes. Awareness only needs to 'proc' once to completely overshadow any defensive benefits of the Slow, and the chance of being critted increases drastically the more mobs you have pummeling you. SF's slow is more window dressing than it is actual mitigation.