Source of magic balance cap assumption

I don't have the data on hand, but from long experience as a mage I can tell you with certainty that yes, in fact, magic balance does cap at 100%. You can verify with JP wiki if you like.

So, even with over 1k int, you still do less than the max dmg almost 50% of the time?

From my own memory I wouldn't say it's 50%. Downward fluctuations in damage happen often, but it's definitely not half the time.

To answer your other question, I'm generally sporting 1k+ Int at any given time while not wearing anything, even a title.

If the occurrence of downward fluctuations is significantly less than half, that would suggest that the cap, if there even is one, is more than 100%.

Could you tell me how often you get less than your max dmg for a spell with a wide min-max range?