I ran some attack bonuses through my spreadsheet because I was curious... keep in mind that some spells may have slightly inflated natural attack bonuses attached, which would increase the value of such an attack increase, but would lower the overall averages shown here.
Format: Attack Bonus Result/No Attack Bonus Result
+75% Blue Magic Attack
P: Lv99, 475 Skill, 171STR, 132DEX, 91VIT, 112AGI, 84INT, 80MND, 80CHR
M: Lv110, 100VIT, 500DEF
No Chain Affinity or Efflux
Heavy Strike: 1260/785
Delta Thrust: 896/442
Vanity Dive: 779/715
Quad. Continuum: 1400/923
Amorphic Spikes: 1145/565
Benthic Typhoon: 1030/508
Chain Affinity (0 and 300)
Heavy Strike: 1849/1152, 2958/1843
Delta Thrust: 1302/642
Vanity Dive: 1107/1015, 1661/1523
Quad. Continuum: 2046/1349, 2865/1889
Amorphic Spikes: 1544/761, 2702/1333
Benthic Typhoon: 1463/721, 1828/902
Efflux
Heavy Strike: 2239/1395
Delta Thrust: 1144/564
Vanity Dive: 1344/1233
Quad. Continuum: 2137/1409
Amorphic Spikes: 2093/1033
Benthic Typhoon: 1498/739
Chain Affinity and Efflux
Heavy Strike: 3063/1908
Delta Thrust: 1550/764
Vanity Dive: 1780/1633
Quad. Continuum: 2912/1920
Amorphic Spikes: 2641/1303
Benthic Typhoon: 1985/979
M: Lv115, 100VIT, 550DEF
No Chain Affinity or Efflux
Heavy Strike: 1027/607
Delta Thrust: 665/192
Vanity Dive: 656/531
Quad. Continuum: 1178/581
Amorphic Spikes: 849/245
Benthic Typhoon: 764/220
Chain Affinity (0 and 300)
Heavy Strike: 1507/891, 2411/1427
Delta Thrust: 966/279
Vanity Dive: 931/754, 1397/1132
Quad. Continuum: 1722/849, 2411/1189
Amorphic Spikes: 1145/331, 2004/579
Benthic Typhoon: 1085/313, 1356/392
Efflux
Heavy Strike: 1825/1080
Delta Thrust: 848/245
Vanity Dive: 1131/916
Quad. Continuum: 1798/887
Amorphic Spikes: 1553/448
Benthic Typhoon: 1111/321
Chain Affinity and Efflux
Heavy Strike: 2497/1477
Delta Thrust: 1150/332
Vanity Dive: 1498/1213
Quad. Continuum: 2450/1209
Amorphic Spikes: 1959/566
Benthic Typhoon: 1472/425
+50% Blue Magic Attack
P: Lv99, 475 Skill, 171STR, 132DEX, 91VIT, 112AGI, 84INT, 80MND, 80CHR
M: Lv110, 100VIT, 500DEF
No Chain Affinity or Efflux
Heavy Strike: 1108/785
Delta Thrust: 738/442
Vanity Dive: 779/715
Quad. Continuum: 1400/923
Amorphic Spikes: 943/565
Benthic Typhoon: 848/508
Chain Affinity (0 and 300)
Heavy Strike: 1626/1152, 2602/1843
Delta Thrust: 1072/642
Vanity Dive: 1107/1015, 1661/1523
Quad. Continuum: 2046/1349, 2865/1889
Amorphic Spikes: 1271/761, 2225/1333
Benthic Typhoon: 1204/721, 1506/902
Efflux
Heavy Strike: 1969/1395
Delta Thrust: 942/564
Vanity Dive: 1344/1233
Quad. Continuum: 2137/1409
Amorphic Spikes: 1724/1033
Benthic Typhoon: 1234/739
Chain Affinity and Efflux
Heavy Strike: 2694/1908
Delta Thrust: 1276/764
Vanity Dive: 1780/1633
Quad. Continuum: 2912/1920
Amorphic Spikes: 2175/1303
Benthic Typhoon: 1635/979
M: Lv115, 100VIT, 550DEF
No Chain Affinity or Efflux
Heavy Strike: 880/607
Delta Thrust: 536/192
Vanity Dive: 656/531
Quad. Continuum: 1129/581
Amorphic Spikes: 685/245
Benthic Typhoon: 616/220
Chain Affinity (0 and 300)
Heavy Strike: 1291/891, 2066/1427
Delta Thrust: 779/279
Vanity Dive: 931/754, 1397/1132
Quad. Continuum: 1650/849, 2310/1189
Amorphic Spikes: 923/331, 1616/579
Benthic Typhoon: 874/313, 1093/392
Efflux
Heavy Strike: 1563/1080
Delta Thrust: 684/245
Vanity Dive: 1131/916
Quad. Continuum: 1723/887
Amorphic Spikes: 1252/448
Benthic Typhoon: 896/321
Chain Affinity and Efflux
Heavy Strike: 2139/1477
Delta Thrust: 927/332
Vanity Dive: 1498/1213
Quad. Continuum: 2348/1209
Amorphic Spikes: 1579/566
Benthic Typhoon: 1187/425
100% of STR = Blue Magic Attack, Ignore 50% of Level Correction
P: Lv99, 475 Skill, 171STR, 132DEX, 91VIT, 112AGI, 84INT, 80MND, 80CHR
M: Lv110, 100VIT, 500DEF
No Chain Affinity or Efflux
Heavy Strike: 1041/785
Delta Thrust: 677/442
Vanity Dive: 955/715
Quad. Continuum: 1310/923
Amorphic Spikes: 866/565
Benthic Typhoon: 779/508
Chain Affinity (0 and 300)
Heavy Strike: 1528/1152, 2445/1843
Delta Thrust: 985/642, 985/642
Vanity Dive: 1355/1015, 2033/1523
Quad. Continuum: 1915/1349, 2681/1889
Amorphic Spikes: 1167/761, 2043/1333
Benthic Typhoon: 1106/721, 1382/902
Efflux
Heavy Strike: 1850/1395
Delta Thrust: 865/564
Vanity Dive: 1646/1233
Quad. Continuum: 1999/1409
Amorphic Spikes: 1583/1033
Benthic Typhoon: 1133/739
Chain Affinity and Efflux
Heavy Strike: 2532/1908
Delta Thrust: 1172/764
Vanity Dive: 2180/1633
Quad. Continuum: 2725/1920
Amorphic Spikes: 1997/1303
Benthic Typhoon: 1501/979
M: Lv115, 100VIT, 550DEF
[spoiler]No Chain Affinity or Efflux
Heavy Strike: 903/607
Delta Thrust: 556/192
Vanity Dive: 855/531
Quad. Continuum: 1090/581
Amorphic Spikes: 710/245
Benthic Typhoon: 639/220
Chain Affinity (0 and 300)
Heavy Strike: 1325/891
Delta Thrust: 808/279
Vanity Dive: 1214/754
Quad. Continuum: 1593/849
Amorphic Spikes: 957/331
Benthic Typhoon: 907/313
Efflux
Heavy Strike: 1605/1080
Delta Thrust: 709/245
Vanity Dive: 1474/916
Quad. Continuum: 1664/887
Amorphic Spikes: 1298/448
Benthic Typhoon: 929/321
Chain Affinity and Efflux
Heavy Strike: 2195/1477
Delta Thrust: 961/332
Vanity Dive: 1952/1213
Quad. Continuum: 2268/1209
Amorphic Spikes: 1638/566
Benthic Typhoon: 1231/425
[/spoiler]
Maximum average increase is +72.44% damage for +75% Blue Magic Attack on the lower level target. I stand by my previous assessment, not really abusive, especially when taking into account recast times, MP costs, and the fact that to make these hit for the same as to half of the damage of a high powered weaponskill
on average requires one or both JAs.