I hope Castablanca won't mind if I chime in a bit.
I understand the weapon throw fully. Makes perfect sense and I've considered it before but I am trying to make my seismic slams as powerfull as possible - I just don't quite have the base damage and crit dmg % that I need. Thus, I use Bash to build it up a bit more, Battle Fury with Marauders. Admiteddly the fury build-up is slow. I was hoping I could sub in Unforgiving and full IK set for a little extra pop eventually.
I've taken a look at your build from your profile and I think your current build is a more burst style build, which means you plan on doing more damage in a short amount of time but it doesn't last very long. As evident from your saying that your fury build-up is slow etc.
I think this is because in an effort to make your slams powerful (as in do a lot of damage), you have actually put together some skills that cause your play style to be a bit choppy. If I'm not wrong, you probably play like this: you engage mobs using Bash to generate fury but it's slow to build it up to full globe, then when you do, you start Slamming but find that you can only slam about 3-5 times and you run out of fury and you have to start using Bash again. So in an effort to increase fury generation, you use Momentum rune for Overpower and Animosity.
I think this mix of skills is actually causing your damage output to be much lower as you can't slam continuously. I think there are two main culprits for that, first is that you're not using Into The Fray for Battle Rage and 2nd, you're not using Rumble for Seismic Slam.
Right now, you are using Bash - Punish, not even Instigation which means you need 12.5 attacks to generate 100 fury (full globe). At an attack speed of 2 attacks per second, that means you need 6 seconds to generate a full globe, which is very slow. Not mention you are using a 2-hander without much IAS on your gear, which means your attack speed is probably less that 2 attacks per second. If you are willing to use Into The Fray, you will find yourself generating fury much faster to start slamming.
Also, I believe you are using Shattered Ground to try and increase your damage output but actually Rumble does more damage than Shattered Ground if the mob stays in the Rumble effect (300% vs 288%). Right now, using Shattered Ground without Into The Fray returns absolutely zero Fury when you slam, hence you can't slam for long. If you're willing to use Rumble with Into The Fray, you will find that you can almost slam non-stop on a good size mob and not having to use Bash at all. That way, you are actually doing much more damage than with Shattered Ground and Marauder's Rage.
Another good thing if you use Rumble and Into The Fray is that you can use Weapon Master Passive with an Axe or Mace (2-hander or 1-handers is your own choice). The extra 10% CC from Weapon Master will make up for the 8% Seismic Slam CC on your IK Boulder Breaker, thus increasing your overall damage for both Seismic Slam and all other attacks as well.
Lastly for your build, you can switch Momentum Rune on Overpower to Killing Spree, that will help increase your damage and help proc Overpower more often and also help return more Fury from Into The Fray.
I think now your main concern of asking about Overpower procs is because you are using the Momentum Rune to help generate fury. If you use the skills I mentioned, you will find that to be a non-issue at all. With 24.5% CC on your gear and with Ruthless and Weapon Master (if you use an Axe or Mace), you will have 44.5% CC which is more than enough to slam non-stop on a mobs, increasing your Slam damage by a lot.
You can use a build like the following:http://us.battle.net/d3/en/calculator/barbarian#aeRQkP!ZYd!bbYZcc
In the above build, I chose WOTB-Thrive on Chaos instead of Insanity, as you can maintain WOTB with just Rumble and Into The Fray. But it's your choice.
Finally, IAS helps a lot more than you think. With faster attack speed, you not just return Fury faster with Into The Fray, but it also helps you maintain the Knockback effect on mobs. The aftershocks of Rumble is spread out over 2 seconds and you do an additional 2 procs (of Into The Fray and LoH) in that 2 seconds. So although Rumble returns more Fury and LoH, you actually have to wait 2 seconds to get the full benefit. If you have faster attack speed, you will have more Rumble effects underneath the mobs at the same time, returning Fury and LoH faster.
Try the build and see how it works for you. I hope this helps.