Friend of me testing it some didn't find a multiplier to be true. It seemed to boost low tier spells a lot more than high tier spells.
Seems to add some amount of Mdamage maybe
I concur with this statement. I did some testing on it today to verify/debunk this theory and came up with these results:
(+8 mDMG JP, +40 natural MAB, +7 MAB gift, +3 MAB from Shiva +1, to cap dINT)
(Nearly) Naked Water I: 259 vs Huge Hornet ((165+8)*1.50=259.5
Same gear, 1000TP Cascade: 409 ((165+8+x)*1.50=409)
Breaking this 409 down, we get:
409/1.50=272.6667
Assuming 273 due to flooring...
273-8=265
265-165=100
Based on this, we see that, at least, @1000TP, you get +100 mDMG.
Same gear, same mob, 2000TP: 559 ((165+8+x)*1.50=559)
Breaking 559 down...
559/1.50=372.6667
Again, assume 373 with flooring...
373-8=365
365-165=200
I can say, pretty conclusively, that at intervals of 1000 TP, you get +100 mDMG.
Now, to see if it's a straight 10% of your TP or if it's a variable...
1500TP: 484
484/1.50=322.6667
323-8=315
315-165=150
That being said, Cascade is 10% of your current TP, converted to mDMG only; no change to MAB.