It doesn’t matter what class combo it is, I just really like the skill. 
Yep and its actually quite busted 
Dual-wield Stormrend Primalist. Literally the most fun ive ever had with a build because the werewolf animation for primal strike is just so juicy and it deals obscene amounts of damage on top (about 5.5 million sustained single target dps with my setup)
Im somewhat too lazy to dig out the complete build right now, so i just scrambled together the core items to give you an idea:
The thing is played with CD-PS but the cooldowns are constantly resetting so its basically a spam-PS without the drawbacks of the node modifier - and the huge bonus on the helmet.
(edit two: this was in reply to this build idea: Druid, Level 100 (GD 1.3.1.0) - Grim Dawn Build Calculator
But the post seems to have disappeared.)
That one has the node penalty, though.
The idea of the berserker version is that you do NOT need the Thunderous Strike node, and thus do NOT get the penalty of -1.4 meter range, -110% weapon damage and -45% TDM.
That makes a huge difference.
And you still lose out on true spam using your max attack speed even when your cooldown is just 0.3 seconds.
Hm… unless… that makes me wonder. how does very short cooldown interact with attack speed?
In Grimtools your druid has an attack speed of 3.24 per second, which is just a bit slower than your cooldown of 0.3 seconds. (0.308 seconds “cooldown” between attacks)
Does that mean that if you assign PS to your left mouse hold it down, as soon as you make your second attack, PS is off cooldown and you get another primal strike attack? If so the cooldown would not matter and you just get the very juicy TDB from the helm. But I’m not sure it works that way and don’t have a build with these exact stats to calculate.
Even if it works this way, you’d need to fine-tune your attack speed very carefully. Suppose you get attacks every 0.29 seconds. Then your second attack would happen BEFORE cooldown ends and would become a normal attack, and you need to wait for your 3rd attack to get another primal strike, and your DPS would drop by half.
Edit: Tested and no, it doesn’t seem to work this way. With a similar 0.3 second cooldown build and only 2 attacks per second, I still only get a primal strike every 2 and sometimes every 3 attacks. The skill visually goes off cooldown, but primal strike still doesn’t trigger immediately.
Without the helmet, every single attack is a primal strike.
With the helmet I can actually make Primal strike hit a bit faster by NOT holding down the button, but by trying to time my attacks to 0.3 seconds. If I’m just a bit too slow or fast it fails, though, and I get a normal attack in between. It’s pretty tricky and requires concentration.
So my conclusion is that the helmet doesn’t really work this way, even at less than 1 second cooldown the penalties of thunderous strike mean you need full spam to make it worthwhile.