The GCD 1 second for OP. OP is by FAR the top DPS because of its crit chance and therefor a 2x multiplier. 2 or 3 OP crits inside of a CS is much better at a 20 or 30 rage cost than one non crit slam and one OP. Especially given that CS will likely be up again quite soon and the slam you just cast will force an excessive .5 second GCD. That being said; if you can get a slam at the opening of a CS while enraged with a HS and both crit, then follow squeaking in two OP crits the numbers can be silly good.

To sum up - Slam and Overpower are not the same either based on a GCD, crit chance or rage cost. Slam is situational, Overpower isn't so much. Slam is sorta the epitome of "if then" statements mathematically speaking. IF you have +40 rage, no stacks in queue for TfB waiting for CS, a target over 20% health, a free 1.5 sec GCD, no free rage abilities or Battleshout and MS is on +1 sec CD THEN Slam. If you're swimming in rage and you have reckbanner or stacks of Gaze (etc) and CS is up I spam slam as well - even to the point of "going oom".

I ref: my posts on the other thread below talking about just this kind of stuff:

http://www.mmo-champion.com/threads/...d-stat-weights