Results of the small test I conducted:
Job: PLD/WAR
Equipment: None
Day: Lightningday
Target: Wild Rabbit
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Banish (no Sibyl): 64
Banish (w/Sibyl): 76
Exact difference: 18.75 MAB
Actual difference: 19 MAB --- floor(64*(1+(19/100))) = 76
-------------------
Banish 2 (no Sibyl): 176
Banish 2 (w/Sibyl): 209
Exact difference: 18.75 MAB
Actual difference: 19 MAB --- floor(176*(1+(19/100))) = 209
-------------------
Holy (no Sibyl): 216
Holy (w/Sibyl): 257
Exact difference: ~18.98 MAB
Actual difference: 19 MAB --- (floor(216(1+19/100))) = 257
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Based on everything here, Star Sibyl appears to grant 19 MAB. This value was consistent from spell to spell and matches the formula for magic damage.