Anyone who has 4 300s and doesn't have the full bonus is getting shafted. I have 5 300s and likely only a 50% boost (seriously, there is no way to tell ATM other than logging into a 300 and calculating the difference).
It would be nice to hear from BioWare about this. Are we being rewarded for grinding CXP, or are we being rewarded for playing different classes? In my humble opinion it looks like it is neither, since the advanced classes are so different from their base class, and yet are being lumped in together.