Not so much a huge difference at the high end, but the lower brackets are closer.
NA Unranked to Silver: 5/10
NA Gold/Platinum: 6/10
NA Diamond: 7/10
NA Crystal: 7.5-8/10
NA Top 30: 8-9/10
JP Unranked to Silver: 6-7/10
JP Gold/Plat: 7-8/10
JP Diamond: 8-8.5/10
JP Crystal: 8.5-9/10
JP Top 30: 9-9.5/10
At the highest levels, there may only be a 0.5-1 gap between regions, but in the lower brackets, you can have a 1-2 difference. The majority of JP is a giant tier above NA, which is the why I compared it to toddlers. Even if you check the "season ending" lodestone messages, back in season 4 they updated matchmaking to make matches more competitive for every data center, but only did it for JP data centers in season 5+.
The majority of NA/EU don't have the pieces put together for the fundamentals etc. The Top 30's on NA during old tournament interviews mostly spoke well of specific jobs from JP data centers that they handled different and also their defensive ability to stall out matches. Frequency of shotcalling is another difference. To not rely on 1-2 people in queue for calls allows a different dynamic when you have multiple ppl pinging and updating markers/drawing attention to things. It's the frequency that you see certain elements of high level play and what skill brackets those elements become "standard" in.
As for the 1shot with MP left argument, the reason why I ignore mana is because when you deal with high lethal matches like Match 3 (which is what happens when you don't have 2 healer comps and pld's nerfing burst), your mana doesn't matter for the 2-3s burst type's. You don't realize you're being focused until 1s into that burst already, once you see either a burst rotation like Emp Arrows from Bard or DRG spear etc. At that point, you have a choice. You can back up, you can try to recupe it back, or you can guard. And so the issue is that most times unless you've been focused already in a match, you're not going to blow guard just from 1 person's damage. You often want to re-position or recupe by default instinctively. That's when the follow-up CC hits and now you're stunned/silenced and unable to re-position or heal, and now you have to purify. By the time you realize your next cast can't go through because of that stun/silence and you need to purify, you're at 2-2.5s in after the purify animation. You're already dead and lucky to get 1 recupe if that. It's one of the scariest situations to be in because after a while, you almost train yourself to blow guard early just to be safe because you know you can't hit it after depending. Your best case for a late guard is you guarding with 20-30% health left and you gotCC waiting for as soon as you get out, and you don't have purify because you burned it just to get the guard off. When you can put out that level of damage, it's a guaranteed kill regardless because you wouldn't have time or the ability to use the mana anyway.