Well I already have over 300% boss, so by replacing my current fafnir staff, I'll still have over 200% boss. But the potential tier is much better since it's a level 160 weapon. So overall I'm replacing Boss% with more Magic Attack and I'm going to try and get MAtk% when I cube it Which means I'll still hit cap on bosses and do more damage on mobs with the Absolab Staff.
EDIT: My staff has 419 m.att.
Assuming you get the 15th star, you'll have 471 m.att.
That's 52 more m.att, but 30% less boss rate.
Keeping in mind that Boss damage lines are the same on 4th tier equips, where is all your boss damage coming from, and how much % boss do you have on your current weapon?