They're already 129% gold efficient. Lowering the cost by 100g without nerfing the stats brings it to 142% and also lets you power spike faster. It might even shift the JG meta to those who benefit most from it. Berserker's would help with clear, dueling and snowballing. ADCs would go for it more too but I don't think that's going to be the primary impact. Udyr, Xin, Noct (he'd be a prime candidate) and probably others may now start taking Berserker's. Those already taking Berserker's, such as Kindred, would of course benefit too. It may even change the value of some mid laners, such as Azir or Yasuo.
Berserker's are not usually a great choice because damage is so high that you need at least defensive boots so that you don't get one shot without being able to output serious DPS. There's no point going full damage if you die nearly instantly and can't use it. Constantly being near a one shot threshold also dramatically reduces the skill and (at least for me), enjoyment that comes with team fighting. Extended fights offer more outplay options and decision making. The meta right now has very few true tanks to provide a frontline and sneaky assassins like Talon are more prevalent too. Berserker's Greaves are not inherently weak. They're just hard to utilize to their true DPS potential.
I think the impact of such a gold reduction may be underrated and its biggest impact may even be in the jungle rather than ADC role. Even if the pick rate becomes more comparable, in another meta, they'd be overtuned. It's sort of a band-aid fix and addresses the high damage (which forces defensive itemization even on carries) by giving them even higher DPS/gold.
I'm not sure what to think of it but I believe the game needs some slight systemic adjustments so that team fighting is somewhat slowed down and more skillful as well as rewarding rather than being so much of a reaction time test. This would have the side benefit of being more beginner friendly and making players feel like they have more control over the game when behind. Of course, if it becomes too slow, the opposite happens and the ahead enemy team just becomes unkillable even when mispositioning. This is just my impression so far and perhaps just having more peel champions in the meta will already basically "fix" this.