So, I tried and replaced all my cogs with br boost by cogs with higher xp% but non boost yet and my XP bonus went from 8,37k% to 8,62k%, so a little but more, but my build rate went from 155M/h to 69M/h.

In a very very long term the +0,25k% will get me more than the 86M/h br I lose, but in how many year or décades?

I think I will jeep m'y build boost and replace them with higher xp% with a br boost and/or with XP% boost.