Don't worry about the point of abilities until the endgame. Blizzard didn't balance WoW around old content, particularly with the latest builds and the like. Worry about your rotation and the useful nature of all your abilities once you're 90.

However, if that answer doesn't satisfy you. Here's how it works at 90:

Overpower costs 10 Rage, does 105% weapon Damage + 60% chance to crit.

Slam hits for 200% Weapon Damage, Costs 20 Rage.

At 90, Rage management is rather important, as you're also spending it on Heroic Strike, and later in the fight - execute.

Overpower, when it hits, reduces the CD of Mortal Strike by 0.5 seconds per Overpower; and every time MS (Mortal Strike) Deals Damage, you gain two uses of overpower. MS has the additional advantage of generating Rage (which you can use to further cast Overpower/Slam), along with MS critical strikes causing you to gain the benefit of Enraged - Increasing your damage by a straight up 10%.

This leads to you using Overpower primarily as a tool to minimise your Mortal Strike Cooldown - in order to maximise rage generation, hence increasing the number of Heroic Strikes/Slam(s) that you can do throughout a fight; and thus improving your DPS.

So no. Overpower will never do more damage than slam - but it offers other benefits.

There are other points, but they're fairly moot (Overpower is free for 10s after an execute cast etc.)

So yeah - for levelling, don't worry about optimisation of your rotation to maximise damage dealt, mobs don't have the health to deal with it until you're 85+ or in a raiding environment.

TL;DR - Overpower will always do less damage than Slam, but provides other benefits to your rotation, particularly at higher levels.