Did some Wizard's Roll testing, this seems like as good a place as any to share the numbers:
Without job bonus:
1: +4
2: +6
3: +8
4: +10
5: +25
6: +12
7: +14
8: +17
9: +2
10: +20
11: +30
bust: -10
I didn't test all numbers with job bonus active, but here are the ones I did:
With job bonus:
2: +16
4: +20
8: +27
10: +30
11: +40
I think we can conclude that the job bonus gives a flat 10 MAB increase.
I used a Pulfanxa to test, comparing base damage to buffed damage and solving the difference for MAB.