Leveling bonus for other classes

I know that you get a bonus of around +40% EXP to any leveling aspect, if you already got another class on Lv 50...

But does anyone know if you also get some bonus if you have let's say 3 classes on Lv 15?

Because I'm planning to level my other 7 classes to Lv 15, then to Lv 30 and then max them out. Would that be slower than maxing them to be Lv 50 one by one?

yep. afaik u get the bonus only of u alrdy hit 50 with one class

It's called the Armoury Bonus. XP bonus amount depends on the difference between the level of your current class, and your highest leveled class.