"XP Bonus" does not apply an experience earning penalty

[temnix]

Some people seem to think that setting a character's experience to 50% will cause him to miss out on half of experience points that come in. In reality he just loses so many points from the deduction, and nothing else.

[Jarno Mikkola]

Well, actually, the bonus is always a multiplier. You set it to be 50%, then you either multiply the "total so far" with a 0.5 with a penalty or 1.5 with a bonus. So yeah, you technically will get the exact same amount of XP as anyone else... and in the game your character creation is even unaffected by it ... so a character with a 100% xp penalty(from a kit effect) will be a level 7 mage at the start of the SoA, and a level 17 mage at the start of the ToB adventure.