Thank you everyone for your thoughts, especially @Amarathiel for all the math :)
I see there are different opinions on this... @Fiana true, I forgot to consider Skill modifier and raid buff. On the other hand, I think that upgrading from BT3->10 for the crux is not much cheaper than 480moon+ely crystals for the early refinement.
I'm convinced, I'll give it a try, like @Crona Darkdance said, it would be the best way to upg for pvp anyway.
@Kozuki I could afford it somehow, but it would be (upgrade costs aside) another thing I'd have to buy from a raid, something I'm not very comfortable with.
@Grimoir That's why I was trying to give some info about crit%, set bonuses, class...I was hoping someone could have a better clue where that 'balance' is.
I'll let you know for science how it goes. :)